Atlanta, GA, USA, 8 August 2016 | AAP

Nick Kyrgios has withstood the intense heat and three-time defending champion John Isner’s serve to win the Atlanta Open for his second ATP World Tour title.

The 21-year-old Australian, 10 years younger than Isner, won 7-6(3) 7-6(4).

Kyrgios matched Isner’s powerful service game, especially in the tiebreakers, and topped the ace count 17-16.

“I knew it was going tough from the get go,” Kyrgios said.

“John just feels so comfortable out on that court.

“I thought in the tiebreaks I didn’t really miss a ball … I just did all the right things. I stayed calm.”

Kyrgios will rise to a career-high ranking of No.16 in the world.

“This week was so much fun,” Kyrgios said. “I had my best friend here. I had my mum here, and a couple of the guys I’m closest with on the Tour were here early in the week.

“It was probably the best week I’ve had off the court in a long time. It’s just rewarding that I played well.”

Kyrgios won in Marseille, France, in February for his lone tour title.

The match ended on Isner’s fourth double fault. During Isner’s 15-match Atlanta winning streak that ended on Sunday, he was 12-2 in tiebreakers.

Kyrgios didn’t appear bothered by the conditions in the match that began with temperatures in the mid-30 degrees celsius, thick humidity and sunshine that helped raise the on-court temperature at Atlantic Station to the mid-50s.

He played crisply, and kept asking for balls quickly when he was serving so he could dictate tempo.

“I wasn’t playing well at all at the start of the week,” Kyrgios, the second seed, said. “I wasn’t in a good place at all (mentally). Some things, I guess you can’t explain.”

Isner often bent over to rest between points and always took the maximum time on changeovers. The 10-time tour winner went through three sweat-soaked shirts to Kyrgios’ one.

“When the sun was out and there weren’t any clouds, I was really struggling,” Isner said. “It was really hot.

“I hadn’t played under the sun this week at all, so maybe it was a bit of an adjustment for me.”

Kyrgios fought off three break points in the second game of the match to even the set at 1-1. Neither player broke the other’s serve all.

“I think him holding that first game certainly was important. It definitely settled him down,” Isner said. “If I go up 2-0, there’s a good chance that set’s over.”

Playing in the day after three night matches, Isner faltered in both tiebreakers.