The commentators who keep bringing up how physically exhausted Isner looks need to figure out that this match has been 100% mental for the last five hours or so. The only relevant body part is the heart.
Serving at 53-54, a loose forehand from Mahut gave Isner 0-30, but Mahut climbed back to 15-30 with some brave serve-and-volleying, and leveled the game at 30-30 with his 85th ace. His 86th made it 40-30, and his 87th gave him the game and another tie at 54-54.
54-54. They've practically each won the set nine times.