HOUSTON (AP) 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.
''Too good,'' the 5-foot-11 Kudla said, shaking his head. ''I tried everything. ... His serve is a joke. ''Some guys can handle it. But I'm not one of the taller guys. It's just enough out of my strike zone for me to do anything about it when he's serving like that.''
In the day's final match, South Korea's Hyeon Chung, 19, defeated 18-year-old American Tommy Paul 5-7, 6-2, 6-2 to become the third teenager since the tournament moved to Houston in 2001 to advance to the quarterfinals.
The first, Andy Roddick, did it in 2001 and 2002 and went on to win the championship both years. Ryan Harrison was the other in 2012.
Chung made it difficult on himself, though. He was a point from taking the first set and let it slip away. But he gave Paul, the French Open junior champion in 2015, no further openings and advanced to his second ATP World Tour quarterfinal.
Juan Monaco, the tournament's 2012 champion who has been bothered by wrist problems, fell out of the top 100 three weeks ago - all the way to 148th - for the first time in nearly a decade and had only three match victories this year before arriving in Houston.
But he looked more like his old self again in a 6-3, 7-5 victory over the second-seeded Benoit Paire of France.
Also, Sam Querrey earned an easy 6-3, 6-3 victory over Germany's Benjamin Becker for his 13th match win in the tournament, the most of any player in the draw. Querrey is through to the quarterfinals for the fourth time and he has been the clay courts runner-up twice.