Alexander Zverev says he struggled with the speed of the court as well as physical problems in his US Open fourth-round defeat by Diego Schwartzman.
The Argentine Schwartzman came from a set down to beat German sixth seed Zverev to reach the US Open quarter-finals at Flushing Meadows.
Schwartzman, the 20th seed, won 3-6 6-2 6-4 6-3 on Monday to set up a potential meeting with three-time champion Rafael Nadal.
The defeat means Zverev, the winner of last year’s ATP Finals, has still only made two Grand Slam quarter-finals – at the French Open in 2018 and 2019.
The 22-year-old said he was hindered by a slow court and an injury suffered in a four-set victory over Aljaz Bedene in the third round two days earlier.
He said: “The court was playing very low and very slow – the opposite to what I prefer.
“I had some things bothering me because of the fall I had two days ago. I couldn’t practise freely, my right hip and back is swollen, but I haven’t suffered any fatigue.
“He [Schwartzman] was playing very aggressive and played very well.
“It’s been a much better week for me than the last two months so I can take some positives.”
Zverev was docked a point for an obscenity in the final set after earlier hitting a ball into the crowd – sparking a lengthy conversation with umpire James Keothavong.
Zverev added: “The court violation was fine but neither of us [Zverev or Schwartzman] heard me getting a warning in the first place.
“The second warning is fine but why a point penalty? I didn’t hear it [the first warning], Diego didn’t hear it, the crowd didn’t hear it. Next time the umpire should let me know.”
After three lengthy sets featuring 10 breaks of serve, Schwartzman, 27, stormed into a 4-0 lead in the fourth against Zverev, who served 17 double faults and made 65 unforced errors.
Zverev clawed a break back but, following a previous code violation for hitting the ball into the crowd, he was then docked a point for an audible obscenity on game point, which gave Schwartzman a 5-2 lead.
Zverev won his next service game but Schwartzman was able to convert the second of two match points with a forehand winner.
Schwartzman will face Nadal or 2014 winner Marin Cilic in the last eight.
“Alex today had many problems with the serve, I am a big returner and I took many chances with that. I think I played great tennis,” he said.