Top seed Ben Shelton met plenty of resistance against Zhang Zhizhen in his opening match at the U.S. Men's Clay Court Championships but ultimately triumphed 7-6 (7-5) 7-6 (7-3).

Neither player broke serve throughout one hour and 53 minutes of action, with Shelton firing off 21 aces to his opponent's 12.

Shelton fell 5-2 behind in the first-set tie-break before rallying to win five consecutive points, and after a series of mammoth service games, he was able to lead the second-set tie-break from the front.

He will face Argentina's Thiago Agustin Tirante in the quarter-finals as he bids to repeat his 2024 triumph in Texas. 

"It was my first match in almost a year on clay," said Shelton, who had not played on the surface since last year's French Open.

"I didn't feel that comfortable playing from the baseline yet, it's just something that's going to take time and take a few matches. 

"I knew I had to be effective with my serve tonight and hit a few aces, and I'm happy that I did."

Shelton was joined in the last eight by fourth seed Tommy Paul, who defeated Paraguay’s Adolfo Daniel Vallejo 3-6 6-4 6-4, having trailed 3-1 in the final set.

Paul will meet Tomas Martin Etcheverry next, after the Argentine defeated Alex Bolt 6-7 (5-7) 6-0 6-3.

Data Debrief: America's number one 

Neither Shelton nor Zhang gave up much on serve. Throughout the match, the American won 72% of his first-serve points, compared to a huge 83% for his opponent.

Where Zhang faltered slightly was on his second serve, with Shelton winning 75% of those points to his opponent's 56%. 

The win was a fitting way for Shelton to mark his ascension to the status of the United States' number one, as he will surpass Taylor Fritz in Monday's ATP World Rankings.