Sixth seed Alexander Zverev narrowly avoided an Australian Open upset as he needed five sets to beat qualifier Lukas Klein and reach the third round.

The Olympic champion, 26, took four hours and 30 minutes to beat Klein, 25, 7-5 3-6 4-6 7-6 (7-5) 7-6 (10-7).

It was only the world number 163's second match in a Grand Slam main draw but the German said Slovakia's Klein "probably deserved to win more".

Second seed Carlos Alcaraz won in four sets against Italy's Lorenzo Sonego.

The Spaniard won a fourth-set tie-break to avoid a fifth set and wrap up a 6-4 6-7 (3-7) 6-3 7-6 (7-3) victory on Rod Laver Arena.

Eleventh seed Casper Ruud was not so lucky as he endured a five-setter against Max Purcell.

Norway's Ruud battled through 6-3 6-7 (5-7) 6-3 3-6 7-6 (10-7) in three hours and 50 minutes to disappoint vocal home support for the Australian.

Both Zverev and Ruud won tense tie-breaks in the deciding set, with the German avoiding a second successive second-round exit in Melbourne.

"I would have much rather won in an hour and a half but what can I do?" he said.

"He played incredible, was hitting every ball as hard as he could from both sides. I didn't really know what to do a lot of the time.

"To be honest, he probably deserved to win more than me. That's how tennis goes sometimes. I'm happy that I'm through but his ranking is no value to how he's playing."

Zverev will play American world number 91 Alex Michelsen in the next round.

Ruud will play British number one Cameron Norrie, who came through his own five-setter on Thursday against Giulio Zeppieri.

"It was an incredible match and it could have gone both ways," Ruud said.

"Max has taken his tennis to a new level and I wish him the best of success in the year to come.

"He plays fast and is one of the most unorthodox players these days. It's tough. He made it hard and luckily in the end it went in my favour."

Alcaraz was tested by blustery conditions and a tricky opponent in Sonego but he was able to keep the world number 46 at bay in the fourth set tie-break to seal victory in three hours and 25 minutes.

It is just the second time Alcaraz, a two-time major winner, has reached the third round in Melbourne, having missed last year's tournament through injury.

"I'm really happy with my performance today," said the Spaniard, who will next face Chinese wildcard Shang Juncheng.

"I think both of us played at a really high level, with high intensity. The match was a little bit tricky with the wind and the sun.

"It was tough to play your best but we tried to stay there all the time, even if I lost the second set."