Rafael Nadal beats Tommy Haas as Roger Federer receives a walkover

Nadal won the Australian Open in 2009

Second seed Rafael Nadal dispelled fears about his physical condition with a 6-4 6-3 6-4 win against Tommy Haas in round two of the Australian Open.

Nadal injured his right knee before the tournament but after his win he said: "I'm very happy about how it improves, I'm able to play without a problem."

Roger Federer also advanced with a walkover after his opponent Andreas Beck withdrew with a back problem.

But eighth seed Mardy Fish was upset in straight sets by Alejandro Falla.

The US number one went down 7-6 (7-4) 6-3 7-6 (8-6) as Falla set up a meeting with Philipp Kohlschreiber.

Fish fell at the same stage in 2010 and 2011, while Colombia's Falla is into round three for the second year running.

There he will face Kohlschreiber, a fellow unseeded player, who advanced when Pere Riba retired while trailing 6-0 4-0.

Next up for Nadal, meanwhile, is Lukas Lacko, of Slovakia, who defeated America's Donald Young 6-3 6-1 3-6 6-3.

"The last couple of years I've had physical problems here," stated Nadal, the champion in 2009 and a quarter-finalist in 2010 and 2011.

"But it's always very exciting to be back here, thanks for supporting me. It's an honour to play on this court [Rod Laver Arena]."

Federer's progress was confirmed when Beck pulled out with a lower back complaint before their encounter on Hisense Arena.

It would have been the first time in 52 matches - a sequence stretching back to 2004 - that Federer had not played on Rod Laver Arena.

The 16-time Grand Slam champion will face Ivo Karlovic, who beat Carlos Berlocq of Argentina 7-6 3-6 6-3 6-4, on Friday for a place in the fourth round.

"I would have loved to have played," said Federer. "I'm sorry to the fans, I was ready to go - I had a warm-up hit and was focused.

"I will rest up, hit intensely [on Wednesday] and get ready for my match."

American 16th seed John Isner won a four-hour 41-minute marathon match, including a 99-minute last set, over former Wimbledon finalist David Nalbandian.

Isner, who won the longest match in history at Wimbledon in 2010, hit 43 aces in his 4-6 6-3 2-6 7-6 (5) 10-8 victory.

Argentinian Nalbandian had several run-ins with chair umpire Kader Nouni and was enraged when supervisor Andreas Egli refused to overrule Nouni's decision to decline a review of a line call late in the fifth.

Under tournament rules, the challenge must be made in a "timely manner'', the judgement on time being at the chair umpire's discretion.

Nalbandian smashed his racket to the ground in disgust when he netted a backhand on match point.

"It's ridiculous playing this kind of tournament with this kind of umpires,'' Nalbandian said. "I didn't understand in that situation, eight-all, break point.''

Alexandr Dolgopolov, the 13th seed from Ukraine, defeated Tobias Kamke of Germany 4-6 6-1 6-1 3-6 8-6.

Dolgopolov next plays Australian teenager Bernard Tomic, who beat American Sam Querrey 3-6 6-3 7-6 (3) 6-3 in the opening night match on Rod Laver Arena.

Seventh seed Tomas Berdych saved the only break point he faced en route to a 6-1 6-1 7-6 (7-4) victory over Olivier Rochus, while Stanislas Wawrinka overcame a third-set wobble to defeat Marcos Baghdatis 7-6 (7-3) 6-4 5-7 6-1 in an entertaining match on Margaret Court Arena.

It takes him to a third-round meeting with 30th seed Kevin Anderson, the South African coming beating Sergiy Stakhovsky 3-6 6-1 7-6 (7-3) 6-3.

Joining them in the third round is 11th seed Juan Martin del Potro - who outclassed Blaz Kavcic 6-4 7-5 6-3 - and his next opponent is Yen-Hsun Lu, a 7-5 6-2 6-2 winner against Florent Serra.