Rafael Nadal began his bid for a men's record 21st Grand Slam title with victory over Marcos Giron in the Australian Open first round.

The Spaniard is making his return to Grand Slam tennis after a five-month layoff with a foot injury.

"I have had very challenging months, tough moments and a lot of doubts," he said on court.

Nadal is the only member of the 'Big Three' competing in Melbourne after nine-time Melbourne champion Novak Djokovic's visa was cancelled on Sunday.

His sole Australian Open title came in 2009 but he won the Melbourne Summer Set in the build-up to the tournament.

He was rarely troubled against Giron, hitting 34 winners to help set up a second-round meeting with German qualifier Yannick Hanfmann who dismissed home favourite and Adelaide champion Thanasi Kokkinakis 6-2 6-3 6-2.

"You never know when you come back from injuries - things are difficult and you go day by day," Nadal, 35, said.

"But I'm staying positive. I won three matches and a title here last week."

Berrettini through even though 'my body wasn't there'

Elsewhere, Wimbledon finalist Matteo Berrettini overcame stomach problems to beat American 20-year-old Brandon Nakashima.

The Italian took a medical timeout during the second set and wrote 'Imodium, grazie' on the camera lens after his 4-6 6-2 7-6 (7-5) 6-3 victory.

"I've been feeling sick with my stomach, let's say that," Berrettini said.

"I really held with my mental strength mostly, because my body wasn't there."

He will play American wildcard Stefan Kozlov in the second round.

World number three Alexander Zverev defeated fellow German Daniel Altmaier 7-6 (7-3) 6-1 7-6 (7-1).

After edging a tight opening set on a tie-break, the Olympic champion gained a double break early in the second set.

Altmaier, 91st in the rankings, broke Zverev's serve in the fifth game of the third set, but the 2020 semi-finalist broke back two games later.

Zverev failed to take four match points in the 12th game but triumphed in another tie-break to set up a second-round tie against Australia's John Millman.

France's Gael Monfils continued his good form with a 6-1 6-1 6-3 win over Argentine Federico Coria and will now face Kazakhstan's Alexander Bublik in the second round.

Rising star Carlos Alcaraz of Spain came through 6-2 6-2 6-3 against Chile's Alejandro Tabilo to set up a meeting with Serb Dusan Lajovic.

Canadian 14th seed Denis Shapovalov beat Serbia's Laslo Djere 7-6 (7-3) 6-4 3-6 7-6 (7-3) and Polish 10th seed Hubert Hurkacz advanced with a 6-2 7-6 (7-3) 6-7 (5-7) 6-3 victory over Egor Gerasimov of Belarus.

Italy's Fabio Fognini faltered against Tallon Griekspoor from the Netherlands as he bowed out 6-4 6-1 6-1.

Two-time Slam finalist Kevin Anderson was another early casualty, beaten by American Reilly Opelka, seeded 23rd, 6-3 6-4 7-6 (7-3).