An injury time goal from Eden Hazard was enough to take Chelsea through 2-1 on aggregate, after drawing with Sparta Prague in London.
The home side looked in control in the early stages at Stamford Bridge and out-of-form striker Fernando Torres saw a couple of his efforts go wide of the target.
However against the run of play Sparta broke the deadlock on the 17th minute when Vaclav Kadlec made a nice run, before pulling the ball back to David Lafata, who picked his spot and slotted the ball into the top corner.
Chelsea could not get a goal themselves before the break but nearly levelled the scores on the night on 47 minutes, when Oscar broke forward and shrugged off a couple challenges, before finding Ramires whose slightly deflected shot cannoned off the crossbar.
Torres had another opportunity moments later when he was through on goal but his flicked finish was blocked by Tomas Vaclik.
On the 65th minute Marek Matejovsky could have made it 2-0 on the night but after he found some space in a dangerous area he couldn't get his shot on target.
Extra-time looked like it was a certainty until substitute Hazard curled a superb shot from the edge of the area with the outside of his boot to score the leveller, to break Czech hearts and take Chelsea through to the next round of the Europa League.