ANDY Murray believes he is returning well enough to meet the giant-serving challenge of John Isner in the fourth round of the Australian Open.

The fifth seed continued his charge through the early rounds with another straightsets victory as he beat Florent Serra 7-5 6-1 6-4.

Murray will next face the 6ft 9in American Isner, who is the second tallest player on the tour and who possesses one of the most feared serves.

Isner, the 33rd seed, booked his place in the fourth round after upsetting 12th seed Gael Monfils 6-1 4-6 7-6 (7/4) 7-6 (7/5).

Murray was faced with a similar task in the opening round when he easily handled Kevin Anderson, the tour’s third tallest player, and he broke the 64th-ranked Serra’s serve on seven occasions.

“I have broken serve a lot so far this tournament against guys that have good serves,”

Murray said.

“Kevin Anderson is a good server. I returned well against him. I’m going to need that in the next match.

“Isner is playing really well.

He won the tournament in Auckland last week.”

In all three of his matches at Melbourne Park so far Murray has broken his opponents in their first service game.

He opened up 5-0 leads in his first two matches and was 3-0 ahead before squandering his lead against Serra.

That allowed the Frenchman back into the set and Murray needed three breaks of serves to clinch it in 53 minutes as he was tested for the first time in the tournament.

Before the event Murray attempted to play down expectations that he could claim his maiden grand slam title, instead claiming he first wanted to reach the fourth round and at least match his best result at the tournament.

With that ambition achieved, the lure of a quarter- final against defending champion Rafael Nadal looms, but Murray steadfastly refused to think past his next task.

“I want to try to get through the next match,” he said.

“Rafa’s got some tough matches to play, as well. I’m sure he won’t be focusing on playing me yet either.”

Defending champion Nadal endured a nervous night on Rod Laver Arena before eventually fending off the persistent challenge of Philipp Kohlschreiber in four sets.

The Spanish second seed was below his best against the 27th-ranked German, whose heavy hitting, especially off the backhand side, caused Nadal problems all night.

In the end Nadal simply took his opportunities better to win 6-4, 6-2, 2-6, 7-5 in three hours and 39 minutes.

The left-hander will play big-serving Croatian Ivo Karlovic in the next round after he defeated countryman and 24th seed Ivan Ljubicic 6- 3 3-6 6-3 7-6 (7/9).

Reigning US Open champion Juan Martin del Potro was also forced to battle as he edged past unseeded German Florian Mayer 6-3 0-6 6-4 7-5 as the temperature in Melbourne peaked at 32 degrees Celsius.

The Argentinian had needed five sets to overcome veteran American James Blake in the previous round and, after being stunned by Mayer in the second set, had to rally from 4- 2 in the fourth to avoid going the distance again.

Del Potro will face a seed for the first time in the fourth round where Marin Cilic awaits after he recovered from losing the first set to 19th seed Stanislas Wawrinka. Cilic won 4-6 6-4 6-3 6-3.

Big-serving American John Isner upset 12th seed Gael Monfils 6-1 4-6 7-6 (7/4) 7-6 (7/5) and he will play Murray in the fourth round.

Isner joined countryman Andy Roddick in the last 16 after the former world number one came from a set down to edge a closely-fought match with Spain’s Feliciano Lopez 6-7 (7/4) 6-4 6-4 7-6 (7/3).

Roddick’s next match is against 11th seed Fernando Gonzalez, the 2007 runner-up, who needed five sets to beat Kazakhstan’s Evgeny Korolev 6-7 (7/5) 6-3 1-6 6-3 6-4 in a match that lasted three hours and 42 minutes.

Reigning US Open champion Kim Clijsters had a night to forget as she crashed out.

The 26-year-old Belgian suffered the worst loss of her professional career as she was thrashed 6-0 6-1 by 19th seed Nadia Petrova under the lights of the Hisense Arena.

There was better news for Clijsters’ countrywomen Justin Henin and Yanina Wickmayer.

Both secured victories to set up an all-Belgian fourthround tie.

Last year’s finalist Dinara Safina was a straight-sets winner over Elena Baltacha, while third seed Svetlana Kuznetsova had to come from a set down to beat unseeded German Angelique Kerber in a match that finished at 1.45am local time.

Eighth seed Jelena Jankovic also crashed out of the women’s draw when she lost to Alona Bondarenko, but the biggest surprise of the day was Clijsters’ capitulation against Petrova.

But Sarah Borwell’s Australia Open run came to an abrupt end yesterday when she lost in the second round of the women’s doubles competition.

Playing with American Raquel Kops-Jones, Middlesbrough born Borwell slipped to a 7-5, 6-2 defeat at the hands of Gisela Dulko and Flavia Pennetta.

By winning in the first round however, Borwell, who is Britain’s highest-ranked female doubles player, retained her place in the world’s top 100.