Rafael Nadal launched his Australian Open title defence with a four-set victory over Britain's Jack Draper as home hope Nick Kyrgios exited through injury without hitting a ball.

In the women's draw, in-form Jessica Pegula and teenage prodigy Coco Gauff were both emphatic winners on the first day of the opening Grand Slam of the year.

Spanish great Nadal, 36, had been in poor form by his sky-high standards, losing six of his last seven matches stretching back to defeat in the last 16 at the US Open.

In fast-rising Draper, 21, the 22-time Grand Slam champion faced a tough test to start his campaign at a sweltering Melbourne Park.

But top seed Nadal recovered from a second-set wobble to defeat the 38th-ranked Briton 7-5, 2-6, 6-4, 6-1 at Rod Laver Arena.

Nadal, who is now a father, faces American Mackenzie McDonald in round two.

"Very exciting, new beginning, just super-happy to be back at Rod Laver with a victory I needed," he said after seeing off Draper.

"Last couple of months have not been easy for me... a very positive start."

Nadal was on court when Australia's talented but temperamental Kyrgios called a hastily arranged press conference and announced that he was out with a knee injury.

"I'm devastated, obviously," said the Wimbledon finalist, who was considered an outside bet to win a maiden Grand Slam crown.

"I've had some great tournaments here, winning the doubles last year and playing the tennis of my life probably going into this event.

"I'm just exhausted from everything, and (it's) obviously pretty brutal."

The Australian Open had already lost several stars in the build-up.

Injured men's world number one Carlos Alcaraz and two-time Melbourne champion Naomi Osaka -- who is expecting her first child -- are among the other high-profile players missing.

Nine-time Australian Open champion Novak Djokovic, who was detained and deported ahead of last year's tournament after refusing to get vaccinated for Covid, begins his title assault on Tuesday.

Other winners in the men's draw on Monday included teenager Shang Juncheng.

The 17-year-old, the youngest player in the men's draw, created a slice of history by becoming the first men's Chinese player to win an Australian Open main-draw singles match.

The teenager battled past Germany's Oscar Otte in nearly three hours and four sets to progress.

He faces American 16th seed Frances Tiafoe next.

Other winners included 18th seed Karen Khachanov, 10th seed Hubert Hurkacz and Italian 15th seed Jannik Sinner. But 21st seed Borna Coric fell to Czech player Jiri Lehecka in straight sets.

Gauff v Raducanu 

In the women's draw, Pegula blitzed 161st-ranked Romanian Jaqueline Cristian on Margaret Court Arena 6-0, 6-1 in a 59-minute romp to signal her intent.

Fellow American Gauff was equally explosive in racing into the second round with a 6-1, 6-4 thumping of Czech Katerina Siniakova on the neighbouring Rod Laver Arena.

Gauff headed into Melbourne on a high after winning her third WTA title at the Auckland Classic, while Pegula was boosted by upsetting world number one Iga Swiatek at the United Cup.

The 18-year-old Gauff now faces a mouth-watering encounter against former US Open champion Emma Raducanu, who beat Tamara Korpatsch 6-3, 6-2, barely 10 days after the Briton exited the Auckland event in tears with an ankle injury.

It will be a first meeting ever between Gauff and the 20-year-old Raducanu, two rising stars of women's tennis.

"I'm really looking forward to this match," Raducanu said.

"I'm very up for it. Coco has obviously done a lot of great things and she's playing well.

"I think we're both good, young players, we're both coming through --  part of the next generation of tennis really -- it's going to be a great match."

Also safely through was another American, Danielle Collins, last year's beaten finalist in Melbourne. She beat Anna Kalinskaya 7-5, 5-7, 6-4.

Greek sixth seed Maria Sakkari also won.

But two seeds fell at the first hurdle, 25th-seeded Czech Marie Bouzkova and American 28th-seed Amanda Anisimova.

Top seed Swiatek gets her campaign for a first Melbourne Park title under way later Monday against Germany's Jule Niemeier.