Top-seeded John Isner had a two-set tournament-record 24 aces to knock out fellow American Denis Kudla in straight sets on Wednesday night in the second round of the US men's claycourt championships in Texas.
The 30-year-old Isner had 12 aces in each set to win 7-6 (7-2) 7-6 (7-4) and was never pushed despite failing to break Kudla's serve in the match.
Isner lost only four of his 46 first-serve points.
"Too good," Kudla said, shaking his head. "I tried everything. 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."
South Korea's 19-year-old Chung Hyeon beat 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 quarter-finals.
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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 quarter-final.
Juan Monaco fell out of the top 100 three weeks ago for the first time in nearly a decade and had only three match victories this year before arriving in Houston.
But the 2012 champion found some form in a 6-3 7-5 victory over second-seeded Benoit Paire of France.
Sam Querrey earned an easy 6-3 6-3 victory over Germany's Benjamin Becker.
