Before the game, Maresca said he was not worried about complacency despite his side holding a two-goal advantage and Enzo Fernandez nearly stretched Chelsea’s lead with a fine 25-yard strike, which was tipped over by Joel Mall after only three minutes.
But, just as in the first leg, Nkunku converted from the penalty spot to seemingly put the tie beyond the Swiss side, although Guillemenot then scored with Servette’s first real chance in the 32nd minute to make it 1-1 on the night.
On a poor quality pitch, Kutesa had a chance to score another for the hosts when he raced through, but, from a tight angle, was denied by Jorgensen.
It has been a great start to the season for Chelsea winger Noni Madueke, who was called up for the England squad for the first time earlier on Thursday after he had grabbed a hat-trick in the 6-2 win over Wolves at the weekend.
He scored the Blues’ second goal in the first leg and looked to have a great chance early in the second half in Geneva, only to be denied by excellent defending.
Crivelli’s header set up a frantic finish before Chelsea’s Nicolas Jackson had a goal ruled out for offside and Cole Palmer hit the crossbar.
In the fourth minute of added time, Servette nearly made it 3-3 overall, but Timothe Cognat dragged his shot wide.