World number one Rafael Nadal reached the third round of the Australian Open with a straight-set win over Argentina's Federico Delbonis.

Nadal, bidding for a record-equalling 20th Grand Slam title, swept aside 76th-ranked Delbonis 6-3 7-6 (7-4) 6-1.

The Spaniard will face compatriot and 27th seed Pablo Carreno Busta next.

There were battling five-set wins for seeds Dominic Thiem and Karen Khachanov on the fourth day in Melbourne, and an entertaining victory for Nick Kyrgios.

Nadal has not won the Australian Open since 2009 but he has entered this year's first Grand Slam as the top seed.

While he was made to work against Delbonis during the second set, the Spaniard had his chances to wrap up the match quicker, converting just three of 20 break points.

Nadal also kissed a ballgirl on the cheek and gave her a headband after he accidentally struck her during a rally.

"I was so scared for her - the ball was quick and straight on to her," Nadal said.

"She's a super brave girl."

Thiem & Khachanov battle through

Russian 16th seed Khachanov fought for four hours and 36 minutes to overcome Sweden's Mikael Ymer.

Khachanov collapsed to the floor - and then struggled to stand with cramp - after sealing a 6-2 2-6 6-4 3-6 7-6 (10-8) victory over his 78th-ranked opponent.

He will face Kyrgios next after the Australian's victory over Gilles Simon.

Fifth seed Thiem came through a five-set thriller of his own against Australian wildcard Alex Bolt.

Two-time French Open finalist Thiem overcame world number 140 Bolt 6-2 5-7 6-7 (5-7) 6-1 6-2.

Thiem, one of the fittest players on tour, ultimately outlasted his opponent in three hours and 26 minutes to set up a meeting with American Taylor Fritz.

"The crowd was very fair. Of course they were for Alex, he's an Aussie, that's completely obvious," Thiem said.

"But it's nice to play in front of a full crowd that's against you than an empty one."

Medvedev & Zverev advance

Fourth seed Daniil Medvedev reached the third round with a straight-set win over Spanish qualifier Pedro Martinez.

The 23-year-old, beaten in September's US Open final, had a nosebleed in the second set but won 7-5 6-1 6-3 on Margaret Court Arena.

Germany's seventh seed Alexander Zverev, who has yet to go beyond the fourth round in Australia, beat Belarusian Egor Gerasimov 7-6 (7-5) 6-4 7-5.

Qualifier Ernests Gulbis of Latvia reached the Melbourne third round for the first time in his career with a straight-set win over Aljaz Bedene.

Gulbis, a former world number 10 who has fallen to 256 in the rankings, won 7-5 6-3 6-2 and will face French 10th seed Gael Monfils next.

Monfils fought back to beat 40-year-old Croatian Ivo Karlovic 4-6 7-6 (10-8) 6-4 7-5.

American John Isner claimed an unusually swift win, taking just 85 minutes to beat Chilean qualifier Alejandro Tabilo 6-4 6-3 6-3.

Big-serving Isner, who has played in the two longest matches in professional tennis history, hit 32 aces.

Kevin Anderson, who played Isner in the record-breaking Wimbledon semi-final in 2018, was beaten 4-6 6-7 (5-7) 7-6 (7-4) 6-2 6-2 by Fritz.