REIGNING US Open champion Juan Martin del Potro survived an epic five-set match against American James Blake to move into the third round of the Australian Open.

The 21-year-old fourth seed had to rally from behind to overcome his older rival 6-4 6- 7 (3/7) 5-7 6-3 10-8 in a match that lasted four hours and 17 minutes under the Hisense Arena lights.

Del Potro’s fellow top seeds had easier paths, with defending champion Rafael Nadal and seventh seed Andy Roddick winning in straight sets.

Del Potro is aiming to become the first player to win his first two grand slams back-to-back, but was nearly derailed by Blake.

Both players finished an energy- sapping encounter suffering with cramp, before Del Potro sealed the match in the 18th game of the fifth set with a forehand down the line.

It was tough on Blake, who was forced to play less than 24 hours after completing his rain-delayed first-round match.

By contrast, Nadal needed only one hour and 53 minutes to brush aside Slovakia’s Lukas Lacko 6-2 6-2 6-2 on the Rod Laver Arena.

It was more convincing display from the Spaniard after his tentative opener against Australian Peter Luczak.

‘‘I think I played the match that I need to play,’’ said Nadal. ‘‘I played more relaxed.

The second round always is easier to play. I think I improved a little bit, but I can still play a little bit longer. It was a very good match.’’ Nadal will meet a seed for the first time in the next round after Germany’s Philipp Kohlschreiber (27) defeated Wayne Odesnik 6-4 3-6 6-3 6-2.

Seventh seed Andy Roddick side-stepped Brazilian Thomaz Bellucci 6-3 6-4 6-4.

Next in his way is Feliciano Lopez, after the Spaniard beat 2003 finalist Rainer Schuettler 6-3 2-6 6-3 6-2.

Fernando Gonzalez, the 2007 winner, and Gael Monfils both won for the second day running. Eleventh seed Gonzalez fired 51 winners as he beat Turkey’s Marsel Ilhan 6- 3 6-4 7-5, while Monfils (12) brushed aside Croatian Antonio Veic 6-4 6-4 6-4.

Monfils ended Irish qualifier Louk Sorensen’s dream run by a 6-3 7-6 (7/4) 7-5 scoreline.