Nantes' winning run at home was ended as Mario Balotelli's 75th-minute winner completed a 2-1 comeback victory for Nice.
Claudio Ranieri's team had won their last five league matches at Stade de la Beaujoire and looked on course to make it six when Yacine Bammou scored after 12 minutes.
However, Alassane Plea equalised just before half-time and Balotelli secured all three points for the visitors with his 13th goal of the season 15 minutes from time.
Minutes after the goal, a pitch invader ran across to pitch towards Balotelli to congratulate him on his goal -- but was wrestled to the ground by stewards.
But Balotelli saw the funny side of the incident, and was all smiles as the fan was taken off the pitch.