Isner, ranked 48, was 4-5 down in the third set with the young Thai No. 2 serving for the match, but managed to overcome his wild card opponent in 2 hours, 12 minutes.
"I knew my opponent is a good player," said Isner, who fired 21 aces. "I got him in the first set, but after that he outplayed me the whole match. And I got a little lucky."
Recalling the crucial match point, he said: "I thought to myself maybe he hadn't been in that situation a lot. He was playing well. I told myself to play some more ball, not making a lot of errors, make him play. See how he reacts serving for the match. I don't think he was playing well that game. After I broke him, I felt really confident."
Isner, who ousted Andy Roddick in the third round of the U.S. Open, will now play either Germany's Mischa Zverev or Leonardo Mayer of Argentina.
In Monday's only other singles match, Brazilian left-hander Thomaz Bellucci beat Guillermo Garcia-Lopez of Spain 6-4, 7-6 (7).
Primera Liga round-upNadal pulls out of Thailand Open
