"I'm exhausted after playing so much over the past four days, having not competed on the hard courts for 18 months", Murray said in a statement.
"It's a very hard position to be coming back from a long injury to be finishing matches at 3 o'clock in the morning".
I don't think I should be put in a position like that.
Murray's withdrawal gave Australia's Alex de Minaur a free pass to the semi-finals where he will meet the victor of Andrey Rublev and American Denis Kudler. Zverev joked after advancing, "I think I'm the only one that's allowed to buy a drink in the USA, right?" I know that the weather's tricky for the scheduling.
"Finishing matches at 3 in the morning is not good", he said, per The Post.
"I don't know how you are expected to recover from that".
After the match, a shattered-looking Murray wept into a towel for several minutes, calling it, "just the emotions coming at the end of an extremely long day and a long match", Murray said.
It has been a tortuous year for the 31-year-old, watching helplessly from the treatment table as he plummeted down the rankings while his hip condition worsened to the extent that surgery was required in January.More news: Bray fire uncovers massive 'EIRE' sign from World War 2
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The three-time Grand Slam champion overcame Romania's Marius Copil last night 7-6, 6-3, 7-6 with the match only finishing at 3.02am local time.
He will instead play next at the Masters event in Cincinnati, which begins a week on Monday, as he continues his build up to what is expected to be his first grand slam tournament since last year's Wimbledon at the US Open later this month.
"I'll try to sleep as late as I can but with your body clock you might get a few hours sleep". When play resumed, Zverev took a second break thanks to a series of errors by Nishikori and closed out the match.
Andy Murray later says he might not play his scheduled last-match quarter-final if he can not recovery sufficiently in time.
On the women's side, two-time Grand Slam victor Svetlana Kuznetsova was to face Kazak eighth seed Yulia Putintseva for a semi-final spot.
The former world number one cited exhaustion for his decision following a gruelling week which had seen him taken to three sets in all three of his matches and spend more than eight hours on court.
After taking a 5-0 lead in the opening-set tiebreaker, Murray dropped seven consecutive points to hand the lead over to Copil.