Isner opened the game at 59-59 with a double fault, and his second serve on the next point barely hit the back of the line.
Two unreturned serves got Isner to 40-15, but two forehand errors from Isner leveled the game at deuce.
Isner hammered an ace up the middle for Ad-In, and then hit a service winner on a body serve to take the first game of the day.
Mahut to serve to stay alive…for the 56th time.