Chelsea blew a golden opportunity to earn a precious first-leg lead in their Champions League last-16 tie against Didier Drogba's Galatasaray.
The Premier League leaders were firmly in command after Fernando Torres's early tap-in gave them a deserved lead.
Torres spurned an early second-half chance to double the advantage, before home skipper Selcuk Inan hit the post.
Chelsea 'paying for not finishing chances off'
But the Blues then conceded a sloppy equaliser as Aurelien Chedjou converted unmarked from close range.
Chelsea were aiming to provide a much-needed boost for English clubs following a miserable fortnight which had already seen
suffer 2-0 losses in their Champions League ties.
The Blues avoided an unwanted quadruple by scoring and avoiding defeat, but will feel they should be heading into next month's return leg with a healthy lead.
Galatasaray boss Roberto Mancini is the more likely to be content with a draw after his team recovered from a shaky opening half-hour during which the visitors threatened to take control of the tie.
Former Chelsea striker Didier Drogba scored 157 goals in 342 appearances for the west London club between 2004 and 2012. He helped the Blues to three Premier League titles, one Champions League, four FA Cups and two League Cups in a glittering eight-year spell.
The Turkish champions went into the match on the back of a 16-game unbeaten run at the Turk Telecom Arena and ex-Manchester City boss Mancini set up his team to make the most of their home advantage.
But they repeatedly surrendered possession cheaply in the opening stages, while maintaining a dangerous high defensive line that Chelsea's pacy attackers looked to exploit on the break.
A trip to Istanbul is still regarded as one of the most uncomfortable in European football, but the home fans were silenced inside the opening 10 minutes by Spain striker Torres.
Galatasaray committed several men forward in attack and, after ex-Arsenal right-back Emmanuel Eboue lost the ball, the visitors countered swiftly down the left, Cesar Azpilicueta squaring for Torres to steer into an unguarded net.
Chelsea continued to put numbers behind the ball after the opener, while still breaking quickly through attacking trio Eden Hazard, Willian and Andre Schurrle.
Didier Drogba struggled to make an impact against his former Chelsea team-mates
Brazil midfielder Willian shot straight at home keeper Fernando Muslera, before Torres had another shot blocked by a last-ditch challenge.
But the game turned on a tactical substitution midway through the first half by Mancini, pitting his wits against Mourinho, the man who
succeeded him at Inter Milan
The Italian sacrificed winger Izet Hajrovic for central midfielder Yekta Kurtulus as he switched to a 4-5-1 formation to provide his team with more stability.
Turkish delight for English clubs
Galatasaray have never won an away tie against English opposition in eight attempts.
27 September 1978:
West Bromwich Albion (L 3-1) Uefa Cup
20 October 1993:
Manchester United (D 3-3) Champions League
7 December 1994:
Manchester United (L 4-0) Champions League
28 September 1999:
Chelsea (L 1-0) Champions League
20 April 2000:
Leeds United (D 2-2) Uefa Cup
20 February 2002:
Liverpool (D 0-0) Champions League
27 September 2006:
Liverpool (L 3-2) Champions League
19 September 2012:
Manchester United (L 1-0) Champions League
It worked perfectly. Chelsea were given less space in midfield and full-backs Azpilicueta and Branislav Ivanovic were prevented from providing extra attacking outlets down the flanks.
Their attacking threat blunted, Chelsea's only second-half chance of note fell to Torres. Hazard threaded a perfectly weighted through-ball to the 29-year-old, whose low shot from 15 yards was acrobatically palmed out by Muslera.
The home crowd, sensing a let-off, started to find their voice as Galatasaray began to grow in belief.
And, just seconds after skipper Inan poked against the post following Drogba's downward header, they found an equaliser.
Neither skipper John Terry, who was rarely troubled by former team-mate Drogba
on his reunion with the Blues,
nor goalkeeper Petr Cech dealt with Wesley Sneijder's left-wing corner, allowing Chedjou to arrive unattended and convert from inside the six-yard box.
How costly that slip proves to Chelsea will be determined in the return leg at Stamford Bridge on 18 March.
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