South Korean teenager Chung Hyeon believes he could have defeated world number five Stan Wawrinka on Thursday if his serving had been better in the match.
The 19-year-old lost the US Open second round encounter in straight sets, but forced each set to a tiebreak and eventually lost 7-6(7-2), 7-6(7-4), 7-6(8-6) after a three-hour battle.
"If I'd been good enough to score aces against Wawrinka from the start, I probably would have won the match," Chung told the Yonhap News Agency.
"I am still trying to improve my serves. It can be intimidating and nerve-wracking to face a top-10 player, but it's something I have to overcome eventually. I am glad I got my lesson early."
"Now that I've played him, I can see why he's the No. 5 player in the world; he does everything so well.
"I felt pressure whenever I served, because I thought I'd have little chance of winning if I couldn't hold my serves against a player like that."
Chung defeated 23-year-old world number 95 James Duckworth, from Australia, 6-3, 6-1, 6-2 in the first round on Monday.
"Before the match, I wanted to pour out my last ounce of energy and play each set for an hour. I didn't want to back down so easily against a strong opponent,” Chung said of his match against Wawrinka.
“I think I stood my ground against Wawrinka because I kept each set going for an hour. But during the match, I just didn't have the presence of mind to think of anything."
Wawrinka, who will face 27-year-old Belgian Ruben Bemelmans in the third round, praised Chung and said the South Korean has a bright future.
"He's a really good player. Was a tough match today. He's moving well. He's a great fighter. He always tried to give you more balls to play or always tried to find a solution.
“He's playing well from the baseline. He can be really good, for sure. He's young and already playing well. He has a long way to go. You have to see how he's going to practice, improve, how he's going to work."