Chelsea must make up a two-goal deficit to stay in the Champions League after Javier Pastore struck late on for Paris St-Germain in their quarter-final.
Ezequiel Lavezzi swept in an early half-volley to put the hosts ahead.
Chelsea responded well, with the impressive Eden Hazard levelling from the spot after a foul on Oscar.
But David Luiz bundled into his own net to restore PSG’s lead and, after Edinson Cavani fired wide, Pastore’s solo effort swung the tie their way.
Deep in injury-time, the Argentine’s mazy run took him from close to the corner flag to the apex of the six-yard box from where he fired a low shot past Peter Cech to transform the tie and the task facing Chelsea at Stamford Bridge.
The Blues, who will be without Ramires after the Brazilian picked up a booking to trigger a suspension, now have to score at least twice in the home leg.