Eight-time champion Novak Djokovic will test his powers of recovery against Alexander Zverev when the Australian Open hits the quarter-final stage Wednesday, while old enemies Serena Williams and Simona Halep also go toe-to-toe.
Djokovic has not practised between matches since injuring his abdomen during a five-set win against Taylor Fritz on Friday and was made to work hard on Sunday by Milos Raonic, dropping another set, before reaching the quarter-finals for the 12th time.
The world number one became only the second man to record 300 match wins in Grand Slam championships after Roger Federer, who has won 362.
Djokovic showed fewer signs of pain against Raonic, but still grimaced after stretching for shots on occasions.
"I had no preparation for this match," said Djokovic, who made it clear that if it was any other tournament than a Grand Slam he would have pulled out.
"I used every hour I had to recover. I have definitely been fitter in the past and I will continue to take each day one at a time."
Djokovic did not train again on Monday, choosing to work on rehab ahead of his clash with Zverev.
"I mean, it's kind of a gamble, that's what the medical team told me," said Djokovic, who is chasing an 18th Grand Slam trophy in Melbourne.
"It's really unpredictable, you can't know what's going to happen with you once you're on the court."
Djokovic beat Zverev in a match that went the distance in the ATP Cup the week before the Australian Open.
"We played an exhausting match in the ATP Cup and I won't expect anything less," said the Serb, who has a 5-2 win-loss record against the German world number 7.
World number two Halep beat Williams 6-2, 6-2 in an unforgettable 2019 Wimbledon final and said she wanted to replicate "the best day of my life" and thwart the American's bid for a record-equalling 24th Grand Slam title once again.
- 'Great to watch, not to play' -
"I felt very confident back then. I need my best tennis to win against her," said Halep, who has lost nine of 11 career matches against Williams.
Opening up the day's action in an-all Asian clash will be 2019 champion Naomi Osaka against the unorthodox Hsieh Su-wei, the oldest woman in the Open era to debut in the quarter-finals of a Slam.
The 35-year-old maverick from Taiwan, described as a "free spirit" by her coach Paul McNamee, seems to get better with age.
She beat eighth seed Bianca Andreescu in the second round at Melbourne for her 16th win against a top-20 players, 10 of them coming in the past two seasons.
The world number 71 has a 1-3 win-loss record against Osaka, the sole victory coming at Miami when the Japanese three-time Grand Slam champion was world number one in 2019.
The match is not a good memory for Osaka.
"If it was a video game, I would want to select her character just to play as her. Because my mind can't fathom the choices she makes when she's on the court," said Osaka.
"It's so fun to watch. It's not fun to play."
The surprise of the men's draw has been Russian qualifier Aslan Karatsev, who will be looking for a semi-final place in his first Grand Slam tournament when he faces Bulgaria's 18th seed Grigor Dimitrov.
Karatsev is one of three Russian men in the last eight of a Slam for the first time since the Open era began in 1968 after compatriots Daniil Medvedev and Andrey Rublev both won in the fourth round on Monday.
The giant-killer became the first player to reach a Grand Slam quarter-final on debut in 25 years when he stunned 20th seed Felix Auger-Aliassime.