Top-seeded John Isner had a two-set tournament-record 24 aces to knock out fellow American Denis Kudla 7-6 (2), 7-6 (4) in the second round of the U.S. men's clay court championships on Wednesday night.

The 30-year-old Isner, ranked 15th in the world, had 12 aces in each set and was never pushed despite failing to break Kudla's serve in the match.

Earlier this year, he had 49 aces in a Davis Cup victory over Australia's Bernard Tomic, when he unleashed a serve clocked at an unofficial record 157 mph.

Isner lost only four of his 46 first-serve points.