Jose Mourinho's men came into Tuesday's home leg of the quarter-final tie trailing 3-1, and an early injury to Eden Hazard looked set to make their task even tougher.
However, the Belgian's replacement - Andre Schurrle - gave the home fans hope by finding the net 14 minutes after coming off the bench.
PSG were forced to make do without talisman Zlatan Ibrahimovic, who limped out of the first leg with a hamstring problem, while Chelsea fans will have been delighted to see a striker at all – Samuel Eto'o back in the starting XI after Mourinho's decision to start without a recognised frontman backfired in Paris.
Mourinho probably should have been the one sweating over his side's future in the competition, but it was opposite number Laurent Blanc who emerged for the first half chewing on a toothpick in an apparent bid to calm any pre-match nerves.
And Chelsea's prospects appeared to suffer a huge blow in the 18th minute when reported PSG transfer target Hazard limped from the field to be replaced by Schurrle.
Lampard was first to really test Salvatore Sirigu in the PSG goal after 28 minutes – his free-kick deflecting off the wall and forcing the Italian into a one-handed diving save.
The German – ineffective as Chelsea's most advanced player in the first leg - went down in the box under the challenge of Marco Verratti during the hosts' next attack, but the referee ignored home claims for a penalty.
It did not appear to be Chelsea's day, but, after Edinson Cavani had wasted a golden opportunity to put the result beyond doubt, Ba became an unlikely hero.
The Senegal striker, who has often been out of favour this season, got the final touch on Cesar Azpilicueta's deflected delivery to divert the ball past Sirigu.
