Serving at 50-50, a couple unforced errors off the Isner forehand gave Mahut 15-40 and double break point.
The last time Isner faced a break point was in the second set, which was over 24 hours ago.
But he dug himself out of the hole valiantly with some massive serving and net play, including a falling overhead to save the second break point.
Isner has survived his first real test of this epic final chapter.
Your move, Mahut.