Bangkok  - France's Gilles Simon and American John Isner continued successful debuts at the Thailand Open Thursday with both advancing in straight sets into the quarter-finals.

Tournament number two Simon, cautious on court as he tries to nurse an injured knee to the season finish line in November, got past American Kevin Kim 6-2, 6-2 with an 11-ace performance.

Eighth seed Isner beat German Mischa Zverev 7-6 (7-5), 6-4 after what he admitted was a sloppy opening set.

"I'm very satisfied to go through," said the 24 year-old ranked 48th with semi-finals at Indianapolis and Washington this season. "After my first round (where he struggled mightily against a teenaged Thai challenger), I'm playing with house money.

"I feel comfortable on the court and I've gotten used to the time change," added the first-time Asian visitor. "At least I'm not falling asleep in the locker room like I did the first few days here.

"This is a slow court but I like it just fine. I feel I can serve through any court," said one of the tallest men in the game at 2.05 metres. "The surface just gives me more time to prepare my shots."

Isner complained about his own performance in the opening set against Zverev, blaming a pair of double-faults for getting him into temporary trouble against the number 50.

"But I played well after that and stuck to my game plan."

Simon said that he also likes the more restrained indoor surface at the Impact arena in contrast to a lightning-fast court.

But the French ATP number ten is more concentrated on his problem knee, which has been bothering him since the spring.

"I had a scan after the US Open and while it's still painful, it's going in a good way - the scan was better than the one I had after Roland Garros," he said.

"I really need a lot of recovery after my matches, it's a lot of icing and rest. Everything depends on my knee."

Simon will play his quarter-final against either American Robby Ginepri or Russian Evgeny Korolev while top-seeded defending champion Jo-Wilfried Tsonga was getting started in the second round against Ernests Gulbis of Latvia. (dpa)