Drained Novak Djokovic admitted he "didn't feel so great" after surviving a physical fourth-round examination against 15th seed Daniil Medvedev at the Australian Open.
The world number one came through an almost three-and-a-half hour, four-set grind that extended into the early hours of Tuesday morning.
Djokovic dropped a set for the second match running but won 6-4, 6-7 (5/7), 6-2, 6-3 at 12:43 am to set up a quarter-final against Japan's eighth seeded Kei Nishikori on Wednesday, 23 January.
Serb Djokovic and Russian Medvedev both needed attention from the trainer during an exhausting encounter liberally sprinkled with punishing rallies of 30 shots or more.
"It was just, you know, a little bit of fatigue, a little bit of back. Nothing major. But there are a couple of things that have surfaced," said the 14-time Grand Slam champion.
"Even though it was three sets to one, it seems like a five-set match, really," said Djokovic. "It was draining physically a lot, because, you know, you just could not rely on the one-two punch tactics."
Djokovic reached his 43rd Grand Slam quarter-final to stay on course for a record seventh Australian Open title, but will next have to conquer marathon man Nishikori.
The Australian Open is Djokovic's most successful major and a seventh crown would put him clear of six-times champion Roger Federer, who was eliminated on Sunday, and Australian great Roy Emerson, whose six wins all came before the Open era.
It would be his third major in a row after winning Wimbledon and the US Open and set him up for a crack at the "Novak Slam".
The French Open comes next and victory at Roland Garros would mean he holds all four majors.
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