Jacob Fearnley claimed he is not a "nervous wreck" before facing world No.3 Alexander Zverev at the US Open after serving 18 double faults in his first round win.
The British No.3 still beat Spanish veteran Roberto Bautista Agut for his first main draw victory in New York.
And the Scot - who took a set off Novak Djokovic at Wimbledon and beat Nick Kyrgios in Australia - has set up another glamorous Grand Slam clash.
But his late-night wobbles against Bautista Agut, in which he was broken serving for the match in the third set after two doubles faults, were a reminder how far and fast the 24-year-old's career has come in the last 15months.
He was ranked outside the top 500 when he left Texas Christian University to turn pro last year and lost in the first round of US Open qualifying to Paul Jubb.
Problems with his serve - and the resulting drop in confidence - have affected the world No.49s form this summer.
"It's definitely nerves," he admitted. "You're gonna double fault. Eighteen is too many obviously. If you look at when the double faults were happening - serving for the match, serving to stay in the set - it's at the highest tension moments. Those ones are definitely down to nerves more than anything.

"I'm not too sure [how to tackle that] . I guess it's helpful to know that even hitting all those double faults I can win the match - that's a positive more than anything. There are good days and bads and in recent matches the doubles faults have been an issue.
"Even though I've had some pretty big match experience. it's still all relatively new for me and being in these in these moments and in these matches still give me a lot of nerves and maybe more than it more than necessarily should. But I'm sounding like I'm a nervous wreck. I'm not. I'm fine. I'm fine.
"I just sometimes in those big moments I need to be a little bit more clear and that will just come with experience. It's not something I'm worried about.
"I know that I have to trust the process and it will get better. I'll be playing Zverev next on another big court, a pretty high stakes, high tense environment and I think putting myself in those situations will help."
Fearnley has yet to take a set off the big-serving German in their previous two meetings in Melbourne and Miami.
"In Miami he played a joke and I didn't feel like I stood a chance," he said. "I think he served like 96% first serve and he doesn't hit the ball softly. It'll be cool and another big challenge to play a guy like him and likely on another big court as well."
Fearnley's opening match was switched to a near empty Grandstand court at 8pm though one American fan made lots of noise - for both players after initially backing the Brit. "When I Iost the service game at 5-4 in the third he completely turned - I couldn't believe it," the Scot said.
"It wasn't abuse, just confusing and extremely loud. He didn't say a nice thing after that!"
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