Isner lost the first point serving at 63-63 on a sloppy looking backhand, but then won four straight to hold for 64-63.
Mahut needed a self-overrule from a line judge on the second point, but after that continued his typical serving to 40-0.
Isner finally stepped in on a second serve to crack a return on the fourth point of game, and a stunned Mahut stuffed his next shot into the net.
But an ace by Mahut on the next serve levels the match again at 64-64.
More returning like he did on that won point could pay big dividends for Isner. He should try it more often.