Les Parisiens looked good for all three points after Javier Pastore opened the scoring on 48 minutes, but a red card to defender Mamadou Sakho 20 minutes from time instigated the turnaround, and Lille subsequently made itsman-advantage count with two goals.
Ancelotti though, has not lost hope in securing the league title in his first season in charge.
“We have to look forward because the championship is not over,” the 52-year-old coach said in his post-match press conference. “It was not a good evening. We have lost an important match which we were in control of.
“After we conceded the penalty and Mamadou Sakho was sent off, things became difficult. I was disappointed with that.
“Lille are now two points behind us, but the title is not lost yet.”
That defeat for PSG sees it stay second, but fall five points behind leaders Montpellier with just four games of the season remaining.