The 2014 US Open finalist, who is seeded eighth, lost the first two sets against Polish qualifier Kamil Majchrzak 6-3 7-6 (8-6).

However, Majchrzak began to suffer cramps on another scorching day in Melbourne and lost the third set 6-0 before retiring in the fourth.

"I have to say he had it today," said three-time quarter-finalist Nishikori.

Majchrzak had never before played in a Tour-level event other than Davis Cup ties, with Nishikori his first top-50 opponent.

"He played very well and it was unfortunate how it finished," added Nishikori. "I didn't do anything, but he was hurt and I feel sorry for him."

Nishikori will now play Croatia's Ivo Karlovic, who recovered from losing the opening set to beat Poland's Hubert Hurkacz 6-7 (5-7) 7-6 (7-5) 7-6 (7-3) 7-6 (7-5).

Meanwhile, fourth seed Alexander Zverev beat Slovenia's former British representative Aljaz Bedene 6-4 6-1 6-4 to progress.

The 21-year-old German saw off the world number 67 in one hour 55 minutes on Rod Laver Arena to set up a second-round meeting with either Jeremy Chardy or Ugo Humbert, both of France.

Croatian Borna Coric, the 11th seed, overcame Belgium's Steve Darcis in straight sets, winning 6-1 6-4 6-4, and will play Hungary's Marton Fucsovics next.

There were also wins for Fabio Fognini and Daniil Medvedev.

Italy's Fognini saw his opponent - Spain's Jaume Munar - retire after the 12th seed won the first two sets on tie-break.

It was far more straightforward for Medvedev, however, as the Russian won 6-1 6-2 6-1 against South African qualifier Lloyd Harris.