Stan Wawrinka admitted he ran out of energy in the Miami Open fourth round as he fell to a three-set defeat to rising star Alexander Zverev on Tuesday.
Wawrinka - the top seed in Miami - had started well on his 32nd birthday and took the opening set 6-4, but that was as good as it got for the Swiss.
Sensing an opportunity, Zverev battled back and after levelling the match completed a superb comeback to claim a second career victory over Wawrinka.
The world number three acknowledged losing his serve early in the second set proved costly as he struggled to find the energy to deny Zverev his last-eight berth.
"One bad game to start the second set and then it was tough for me," Wawrinka said.
"I completely went down physically and mentally, and also my tennis was completely out. I had no more gas."
For Zverev it is a first appearance in the quarter-finals of an ATP 1000 event, where he will meet Nick Kyrgios on Thursday.
The 19-year-old was delighted to have ousted the top-ranked player at the tournament, and set his sights on a tough clash with the in-form Kyrgios.
"It's great, I've beaten great players to get here," he told the ATP website.
"I didn't have an easy draw at all with [John] Isner in the third round, I played really well yesterday [Monday] and then Stan the number one seed.
"I played a great match, after the first set I think I played maybe one of the best tennis matches of my life so I'm very happy, I look forward to the quarter-final and hopefully I can win a few more matches here.
"Nick played really well in Indian Wells, he had a very tough draw and got through that until he got sick, he's playing great tennis, it's not going to be easy for me."