Chelsea's hopes of qualifying for the Champions League suffered a setback as Jay Rodriguez and Rickie Lambert scored to earn struggling Southampton victory.
Rodriguez opened the scoring with a composed 23rd-minute finish after good work by Lambert and SteveN Davis.
Southampton deserved win - Pochettino
John Terry marked his first league start since 2 February by heading a 33rd-minute equaliser from a corner.
But Lambert's 25-yard free-kick earned the Saints victory as Chelsea suffered a third straight away league defeat.
Southampton climbed to 13th in the table after establishing a four-point safety cushion from the relegation places, with Mauricio Pochettino's side recording back-to-back league wins for the first time since November.
But while the Saints appear to have hit form at the right time, the same cannot be said of Chelsea.
With visits to Liverpool and Manchester United on the horizon, Blues interim boss Rafael Benitez must find a solution for his team's travel problems if they are to cement a top-four finish.
Graeme Le SauxFormer England defender on BBC Radio 5 live
Southampton fans are singing 'Lambert for England'. The only thing about that is, he is not particularly mobile and not one of those strikers who can get away quickly from an opponent. The international game doesn't always suit a player like that.
Chelsea dropped to fourth in the table, just two points ahead of fifth-placed Arsenal.
This was not the way Benitez intended to start Chelsea's most crucial spell of the season.
With an FA Cup tie against Manchester United on Monday, Chelsea made seven changes to the side that overcame West Ham on their last Premier League outing.
Yet Benitez will have expected much more from those on duty, as Southampton, who had 24 points fewer than their opponents before the start, showed more hunger and desire.
Chelsea, who also face a crucial Europa League game against Rubin Kazan on Thursday, failed to heed the warning signs even after Rodriguez produced a fine finish following a clever build-up involving Davis and Lambert.
The lead lasted just 10 minutes as Terry was allowed to head his second league goal of the season following poor defending from a corner.
That should have awoken Chelsea from their slumber but the Saints regained the initiative through a sublime 35th-minute Lambert strike.
Benitez upset with Chelsea defending
The 31-year-old former Bristol Rovers and Rochdale striker marked the 500th league appearance of his career with a terrific curling free-kick which Chelsea keeper Petr Cech could do little about.
Saints fans responded with chants of "Lambert for England" after the striker took his Premier League goal tally for the season to 14.
The home side was forced to bring on substitute goalkeeper Kelvin Davies at the start of the second-half after Artur Boruc complained of dizziness.
But Chelsea still could not muster an equaliser, Victor Moses coming closest with a swerving effort which former Sunderland stopper Davies did well to tip over the bar.
Southampton manager Mauricio Pochettino:
"Victories like today prove that we are on the right path.
"It is very important for the confidence of my players and especially the people surrounding our club, the technical staff.
"It is important that we believe we can achieve results as we saw today and we can continue achieving them in the future."
Chelsea interim manager Rafael Benitez:
"I think that you could see that in the first half we didn't have the intensity we were expecting.
"We improved in the second half, made more chances, but still we didn't make the right decisions in the final third.
"But at least we were there and made the chances."
Full Time The final whistle is blown by the referee.
Unfair challenge on Morgan Schneiderlin by Yossi Benayoun results in a free kick. Direct free kick taken by Morgan Schneiderlin.
Free kick awarded for an unfair challenge on James Ward-Prowse by Nascimento Ramires. Free kick taken by Morgan Schneiderlin.
Foul by Luke Shaw on Nascimento Ramires, free kick awarded. Free kick taken by Nascimento Ramires.
Cesar Azpilicueta crosses the ball, Daniel Fox gets a block in.
Unfair challenge on Nascimento Ramires by Morgan Schneiderlin results in a free kick. Free kick taken by Nascimento Ramires.
Direct effort from the free kick comes in from Frank Lampard.
Booking Morgan Schneiderlin is shown a yellow card.
Morgan Schneiderlin gives away a free kick for an unfair challenge on Eden Hazard.
Rickie Lambert is ruled offside. Branislav Ivanovic takes the free kick.
Substitution Steven Davis goes off and Daniel Fox comes on.
The ball is crossed by Victor Moses, blocked by Luke Shaw.
Booking Rickie Lambert receives a caution for unsporting behaviour.
Unfair challenge on Fernando Torres by Jos Hooiveld results in a free kick. Frank Lampard fires a strike on goal direct from the free kick, Jay Rodriguez gets a block in.
The referee gives a free kick against Rickie Lambert for handball. Branislav Ivanovic takes the free kick.
Kelvin Davis takes the free kick.
Substitution Yossi Benayoun is brought on as a substitute for Emboaba Oscar.
Booking Fernando Torres is given a yellow card.
Free kick awarded for an unfair challenge on Maya Yoshida by Fernando Torres.
The ball is sent over by Nathaniel Clyne.
Free kick awarded for an unfair challenge on Ryan Bertrand by Jay Rodriguez. Petr Cech takes the free kick.
Substitution Nascimento Ramires on for Mikel.
Free kick awarded for a foul by Rickie Lambert on Mikel. Mikel takes the free kick.
Header by Eden Hazard from deep inside the penalty area misses to the right of the target.
Inswinging corner taken right-footed by Steven Davis, Headed effort on goal by Jos Hooiveld from inside the penalty box misses to the right of the goal.
Luke Shaw takes a shot. Branislav Ivanovic gets a block in.
Jay Rodriguez takes a shot. Save made by Petr Cech.
Victor Moses decides to take a short corner.
The ball is crossed by Cesar Azpilicueta, Jack Cork makes a clearance.
Corner taken by Steven Davis from the right by-line to the near post, Header from close range by Jay Rodriguez misses to the right of the target.
The referee blows for offside. Indirect free kick taken by Maya Yoshida.
Substitution James Ward-Prowse replaces Jason Puncheon.
Substitution Eden Hazard comes on in place of Marko Marin.
Effort from the edge of the penalty box by Jack Cork goes wide of the right-hand upright.
Victor Moses takes a shot. Kelvin Davis makes a save. Short corner taken by Marko Marin.
Free kick awarded for a foul by Morgan Schneiderlin on Fernando Torres. Free kick taken by Fernando Torres.
Nathaniel Clyne sends in a cross.
The referee blows for offside against Rickie Lambert. Petr Cech takes the indirect free kick.
Marko Marin takes a shot. Maya Yoshida gets a block in.
Frank Lampard is caught offside. Free kick taken by Kelvin Davis.
Corner taken by Marko Marin, Jay Rodriguez makes a clearance.
Unfair challenge on Ryan Bertrand by Jason Puncheon results in a free kick. John Terry takes the direct free kick.
Outswinging corner taken by Steven Davis from the right by-line, Header by Jos Hooiveld from deep inside the penalty area misses to the left of the goal.
Luke Shaw fouled by Cesar Azpilicueta, the ref awards a free kick. Free kick taken by Steven Davis.
The second half kicks off.
Substitution Artur Boruc goes off and Kelvin Davis comes on.
45:00 +2:02 Half time
Half Time The ref blows to signal half-time.
Centre by Jack Cork, clearance made by John Terry.
Outswinging corner taken by Steven Davis, clearance by Ryan Bertrand.
Free kick awarded for an unfair challenge on Cesar Azpilicueta by Jay Rodriguez. Cesar Azpilicueta restarts play with the free kick.
The referee blows for offside. Petr Cech restarts play with the free kick.
A cross is delivered by Nathaniel Clyne, comfortable save by Petr Cech.
Jay Rodriguez takes a shot. Branislav Ivanovic gets a block in.
Victor Moses is penalised for a handball. Morgan Schneiderlin takes the indirect free kick.
Effort on goal by Mikel from just inside the area clears the bar.
34:05 Goal scored
Goal! - Rickie Lambert - Southamp'n 2 - 1 Chelsea Rickie Lambert hits the back of the net with a goal direct from the free kick from outside the area to the top left corner of the goal. Southampton 2-1 Chelsea.
Free kick awarded for a foul by Ryan Bertrand on Steven Davis.
Assist by Marko Marin.
32:06 Goal scored
Goal! - John Terry - Southamp'n 1 - 1 Chelsea John Terry grabs a headed goal from just inside the box. Southampton 1-1 Chelsea.
Corner taken by Marko Marin,
The referee blows for offside against Ryan Bertrand. Artur Boruc restarts play with the free kick.
Steven Davis gives away a free kick for an unfair challenge on Ryan Bertrand. Frank Lampard restarts play with the free kick.
Corner taken right-footed by Emboaba Oscar from the left by-line to the near post, Morgan Schneiderlin manages to make a clearance.
Fernando Torres is penalised for handball and concedes a free kick. Free kick taken by Artur Boruc.
Jay Rodriguez gives away a free kick for an unfair challenge on Cesar Azpilicueta. Mikel restarts play with the free kick.
Assist on the goal came from Steven Davis.
22:47 Goal scored
Goal! - Jay Rodriguez - Southamp'n 1 - 0 Chelsea Jay Rodriguez gets on the score sheet with a goal from inside the box to the bottom right corner of the goal. Southampton 1-0 Chelsea.
Foul by Frank Lampard on Jack Cork, free kick awarded. Jack Cork takes the free kick.
Jack Cork concedes a free kick for a foul on Victor Moses. Petr Cech takes the direct free kick.
Corner from right by-line taken by Steven Davis.
Rickie Lambert produces a cross, blocked by Emboaba Oscar.
Outswinging corner taken right-footed by Steven Davis from the right by-line, clearance by John Terry.
The ball is crossed by Nathaniel Clyne, Ryan Bertrand gets a block in.
Jason Puncheon crosses the ball, clearance by John Terry.
Outswinging corner taken right-footed by Steven Davis, clearance made by Mikel.
Rickie Lambert takes a shot. Branislav Ivanovic gets a block in.
Jay Rodriguez takes a shot. Save by Petr Cech.
Corner taken right-footed by Marko Marin to the near post, clearance by Luke Shaw.
Steven Davis takes a shot. Blocked by Frank Lampard.
Marko Marin takes the outswinging corner, Jay Rodriguez makes a clearance.
The ball is sent over by Victor Moses.
Steven Davis produces a left-footed shot from the edge of the box and misses to the right of the target.
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