Russian third seed Daniil Medvedev survived a scare as he fought back from behind before French opponent Terence Atmane quit in tears because of cramp in the Australian Open first round.
Qualifier Atmane won the opening set before Medvedev - runner-up in 2021 and 2022 - turned the match around.
Medvedev led 5-7 6-2 6-4 1-0 when Atmane was forced to retire.
"I've cramped like that many times. If he plays like this he can do big things," Medvedev, 27, said.
The conditions in Melbourne on the second day's play were hotter than they have been over the past week, with outside temperatures reaching 27C and increasing further in direct sunlight on court.