Ramires scored twice in two minutes in the closing stages as Chelsea booked a place in the FA Cup fourth round.
Andre Villas-Boas' side struggled for a cutting edge against Championship side Portsmouth at Stamford Bridge but took the lead when Juan Mata swept in from close range just after half-time.
Ramires poked in at the far post, and then finished off a counter-attack.
Frank Lampard added a fourth from the edge of the area late on to give the scoreline a flattering gloss.
Ramires is hardly known as a prolific goalscorer, but his late double takes his tally to eight for the season - making him the club's third top-scorer.
It was the first time Chelsea have managed four goals since their 5-0 victory over Genk in the Champions League in October.
But it was not a reflection of the way they played and there was yet more frustration for Fernando Torres.
With Didier Drogba away at the Africa Cup of Nations with Ivory Coast for much of January, Torres looks set to get a run in the Chelsea side.
But, looking for a first goal since October, he was starved of service throughout and only threatened on one occasion, when he did well to force a save from Stephen Henderson with a header after being picked out by Florent Malouda on the edge of the area.
Mata's sixth goal since joining in a £23.5m deal from Valencia in the summer set the Blues on their way to victory.
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Ramires' tally of eight goals has already beaten his previous best (seven goals for Cruzeiro in 2008/09) for an entire campaign.
But against a team struggling in the second tier, Chelsea would have been expected to trouble Henderson far more than they did before a flurry of three goals in eight minutes late on.
Dave Kitson went close to opening the scoring with a drive that flashed just wide of goal with less than 30 seconds gone.
Aaron Mokoena, one of three survivors from the Portsmouth team that was beaten 1-0 by Chelsea in the final in 2010, missed with a header from a Liam Lawrence corner.
After failing to turn their possession into any more chances in a forgettable first half, the Blues grabbed the lead just two minutes after the interval.
Malouda, who was making his first start since November, embarked on a powerful run down the left flank and his low cross was calmly finished from close range by Mata.
But Portsmouth had several chances to pull level in a scramble after David Luiz gave the ball away.
Petr Cech saved from Marko Futacs and John Terry, who required treatment after colliding with the post, blocked from Ricardo Rocha before Cech denied Futacs once again.
Chelsea looked likely to limp to victory before Mata's cross flew over Torres and Jason Pearce for Ramires to stab in with four minutes left.
Two minutes later, the hosts broke swiftly and Ramires touched in from a Torres assist.
And three minutes into stoppage time, Ramires contributed the final pass before Lampard drove in from the edge of the area for his tenth goal of the season.
Chelsea assistant manager Roberto Di Matteo:
"I thought it was a very good all-round performance. Portsmouth made it very hard, especially in the first 45 minutes, and it is always difficult to break an opponent down and find a space with so many players.
"It was just a matter of scoring the first goal and we were always confident we were going to be able to do that.
"In the second half we got further forward and secured the win. It is a competition we would like to win and today we have gone a step further towards the target."
Portsmouth manager Michael Appleton:
"Conceding after two or three minutes of the second half does not help matters, especially after a good first-half performance.
"I thought we got back into the game and you could sense there was a bit of nervousness before the end when they got their second goal.
"I think the performance was a credit to the boys but that is why Chelsea are where they are and we're a Championship club."
Live Text Commentary
Full Time The referee blows his whistle to end the game.90:00 +2:31
Assist by Ramires.90:00 +2:31 Goal scored
Goal! - Frank Lampard - Chelsea 4 - 0 Portsmouth Frank Lampard scores a goal from just inside the penalty area to the bottom left corner of the goal. Chelsea 4-0 Portsmouth.90:00 +2:20
A cross is delivered by Ramires, save made by Stephen Henderson.90:00 +1:13
Shot comes in from Greg Halford from the free kick, Dave Kitson gets a block in.90:00 +1:13 Booking
Booking Raul Meireles booked for unsporting behaviour.90:00 +1:09
Raul Meireles challenges Marko Futacs unfairly and gives away a free kick.89:25
Juan Mata takes the outswinging corner, clearance made by Greg Halford.89:14
The ball is sent over by Romelu Lukaku, clearance made by David Norris.88:38
Romelu Lukaku gives away a free kick for an unfair challenge on Ryan Williams. Stephen Henderson takes the direct free kick.87:22 Substitution
Substitution Fernando Torres leaves the field to be replaced by Romelu Lukaku.86:42
The assist for the goal came from Fernando Torres.86:42 Goal scored
Goal! - Ramires - Chelsea 3 - 0 Portsmouth Ramires finds the back of the net with a goal from deep inside the penalty box to the top right corner of the goal. Chelsea 3-0 Portsmouth.85:15 Substitution
Substitution Ryan Bertrand is brought on as a substitute for Ashley Cole.84:40
Assist by Fernando Torres.84:40 Goal scored
Goal! - Ramires - Chelsea 2 - 0 Portsmouth Ramires fires in a goal from inside the six-yard box to the bottom left corner of the goal. Chelsea 2-0 Portsmouth.83:58
The ball is sent over by Juan Mata. Fernando Torres takes a shot.81:36
Frank Lampard takes a short corner.80:51
Juan Mata restarts play with the free kick.80:51 Booking
Booking Caution for Dave Kitson.
Free kick awarded for a foul by Dave Kitson on Juan Mata.79:31
Shot by Ryan Williams from outside the box goes over the target.79:07
Centre by Juan Mata, Jason Pearce makes a clearance.77:10 Substitution
Substitution Florent Malouda leaves the field to be replaced by Oriol Romeu.76:49
Juan Mata produces a cross, Jason Pearce manages to make a clearance.75:19
Florent Malouda takes the direct free kick.75:19 Booking
Booking Booking for Ryan Williams for unsporting behaviour.74:54
Ashley Cole fouled by Ryan Williams, the ref awards a free kick.74:24
Free kick taken by Liam Lawrence.74:24 Booking
Booking The referee shows John Terry a yellow card for unsporting behaviour.73:42
John Terry challenges Ryan Williams unfairly and gives away a free kick.72:15
Corner from the right by-line taken by Liam Lawrence, Jason Pearce has a headed effort at goal from close in which goes wide right of the target.69:50 Substitution
Substitution Ryan Williams is brought on as a substitute for Ricardo Rocha.66:04 Substitution
Substitution Erik Huseklepp is brought on as a substitute for Hayden Mullins.65:57
The ball is crossed by Ashley Cole.63:58
The ball is crossed by Florent Malouda, Stephen Henderson makes a save.59:56
Ashley Cole produces a cross. Ramires takes a shot. Save by Stephen Henderson.59:02
Inswinging corner taken left-footed by Juan Mata, Dave Kitson manages to make a clearance.58:51
A cross is delivered by Jose Bosingwa, clearance by Jason Pearce.58:26
Centre by David Luiz, clearance made by Dave Kitson.57:20
Frank Lampard takes a shot. Hayden Mullins gets a block in. Outswinging corner taken from the left by-line by Florent Malouda, Dave Kitson manages to make a clearance.56:31
Foul by John Terry on Marko Futacs, free kick awarded. Hayden Mullins restarts play with the free kick.53:01
Marko Futacs takes a shot. Petr Cech makes a save. Ricardo Rocha takes a shot. Blocked by John Terry.52:14
David Luiz delivers the ball, clearance by Jason Pearce.50:46
Short corner taken by Florent Malouda from the left by-line.49:28
Free kick awarded for an unfair challenge on David Norris by David Luiz. Liam Lawrence fires a strike on goal direct from the free kick.48:33
The ball is delivered by Joel Ward, save by Petr Cech.47:20
Florent Malouda provided the assist for the goal.47:20 Goal scored
Goal! - Juan Mata - Chelsea 1 - 0 Portsmouth Juan Mata finds the net with a goal from inside the six-yard box to the bottom left corner of the goal. Chelsea 1-0 Portsmouth.46:52
The ball is crossed by Florent Malouda,46:43
The ball is swung over by Florent Malouda, clearance by Jason Pearce.45:01
The referee gets the second half underway.45:00 +1:49 Half time
Half Time The ref blows to signal half-time.45:00 +1:03
Unfair challenge on Florent Malouda by David Norris results in a free kick. Frank Lampard has an effort direct from the free kick.45:00 +0:31
Foul by Joel Ward on Ashley Cole, free kick awarded. David Luiz takes the free kick.44:20
The ball is swung over by Juan Mata, clearance by Ricardo Rocha.43:34
Free kick crossed left-footed by Juan Mata from right wing, Jason Pearce manages to make a clearance.43:34 Booking
Booking Greg Halford receives a yellow card.43:08
Greg Halford concedes a free kick for a foul on Jose Bosingwa.41:00
The assistant referee signals for offside against Fernando Torres. Indirect free kick taken by Stephen Henderson.39:22
Stephen Henderson restarts play with the free kick.39:22 Booking
Booking Booking for Florent Malouda.39:17
Free kick awarded for an unfair challenge on Joel Ward by Florent Malouda.38:52
Foul by Greg Halford on Frank Lampard, free kick awarded. Free kick taken by Jose Bosingwa.38:10
Inswinging corner taken right-footed by Frank Lampard from the left by-line, clearance made by Dave Kitson.37:00
Juan Mata crosses the ball in from the free kick, Ricardo Rocha makes a clearance.37:00 Booking
Booking Jason Pearce is booked.36:52
Free kick awarded for an unfair challenge on Fernando Torres by Jason Pearce.34:03
The ball is crossed by Florent Malouda, Greg Halford makes a clearance.33:19
David Norris has a shot on goal from outside the area which misses right.32:02
Ramires takes a long range shot clearing the crossbar.31:35
Marko Futacs gives away a free kick for an unfair challenge on John Terry. Raul Meireles takes the free kick.29:43
The ball is swung over by Florent Malouda, Aaron Mokoena gets a block in.28:38
The ball is sent over by Juan Mata.24:29
The ball is crossed by Florent Malouda, Ricardo Rocha manages to make a clearance.24:13
The ball is sent over by Liam Lawrence, clearance by David Luiz.22:05
Juan Mata produces a left-footed shot from just outside the penalty area that goes harmlessly over the target.21:36
The assistant referee flags for offside against Marko Futacs. David Luiz takes the indirect free kick.18:33
Corner taken right-footed by Liam Lawrence, Aaron Mokoena produces a header from inside the six-yard box that clears the bar.17:59
The ball is crossed by Joel Ward, John Terry makes a clearance.15:51
Free kick awarded for a foul by David Norris on Frank Lampard. Direct free kick taken by David Luiz.14:27
Foul by David Norris on Raul Meireles, free kick awarded. Free kick taken by David Luiz.13:36
The ball is sent over by Florent Malouda.12:15
Inswinging corner taken by Juan Mata from the right by-line played to the near post, David Luiz has an effort at goal from close in which goes wide right of the goal.12:00
The ball is delivered by Florent Malouda, Fernando Torres takes a shot. Save made by Stephen Henderson.10:36
Florent Malouda delivers the ball, Stephen Henderson makes a comfortable save.9:41
Raul Meireles takes a shot. Ricardo Rocha gets a block in.8:53
Centre by Jose Bosingwa.5:28
A cross is delivered by Juan Mata, clearance made by Aaron Mokoena.5:04
Jose Bosingwa delivers the ball, Ricardo Rocha makes a clearance.2:11
Unfair challenge on David Luiz by Marko Futacs results in a free kick. David Luiz restarts play with the free kick.0:29
Effort from inside the area by Dave Kitson misses to the right of the target.0:00
The referee gets the match started.
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