Murray safely through to last 16

Andy Murray faces a tough test in round four of the Australian Open after winning through to the last 16 with a straight-sets victory over Florent Serra.

The British number one and tournament fifth seed has not dropped a set so far in his three matches, and beat Frenchman Serra 7-5 6-1 6-4 on Friday.

His next opponent will be big-serving American John Isner, who caused a minor upset by knocking out 12th seed Gael Monfils in four sets.

Isner is a player in form, having won the Heineken Open in Auckland last week. The giant American was also the man responsible for eliminating Ireland's Louk Sorensen in the previous round.

Murray said: 'He gives guys a lot of problems. He's 6ft 9in with a huge serve.'

The 22-year-old had only been broken twice in his first two matches but saw Serra twice break back in the opening set as the Scot was put under pressure for the first time in the tournament.

Murray responded to the challenge emphatically as he broke Serra for a third time to take the opening set in 52 minutes.

Murray then raced through the second set and when he broke Serra's opening two service games in the third, the result had suddenly become inevitable.

Murray's victory means he fulfilled his modest pre-tournament ambition of matching his best result in Australia, the fourth round.

Isner blasted 26 aces past Monfils, the 12th seed, in their third-round match as the American edged out his French opponent 6-1 4-6 7-6(7/4) 7-6(7/5) on Margaret Court Arena.

Fourth seed and US Open champion Juan Martin del Potro overcame a tough challenge from Florian Mayer to win in four sets.

The 21-year-old served 11 aces but was also guilty of making 46 unforced errors in an inconsistent display.

Mayer took all three of his break point chances to win the second set to love in only 28 minutes.

Del Potro was forced to save two break points in the final game but sent down an unreturnable serve to win 6-3 0-6 6-4 7-5 and will now face either Marin Cilic or Stanislas Wawrinka.

World number seven Andy Roddick fixed up a meeting with Fernando Gonzalez after edging out Feliciano Lopez in four tight sets 6-7(4) 6-4 6-4 7-6(3).

Lopez took a set off the American for just the second time in six meetings when he won the opener on a tie-break.

However, the Spaniard was undermined by a high number of unforced errors of 60 including 21 in the final set.

Lopez produced a string of forehand errors in the fourth set tie-break to bring up five match points and although he saved two, Roddick went through after his opponent put a return into the net.

His next opponent Gonzalez was on court for three hours and 22 minutes as he edged out Evgeny Korolev 6-7(7/5) 6-3 1-6 6-3 6-4.

After levelling the match at two sets all, Gonzalez claimed the vital break to edge out his opponent who is the first Kazakh man to reach the third round of a grand slam.

Roddick has an 8-3 career record against Gonzalez but he is taking nothing for granted.

'He is a very dangerous player,' said the 2009 semi-finalist.

'Some days he comes out and looks like he is playing ping-pong the way he can sling the ball around.'

Defending Australian Open champion Rafael Nadal endured a nervy 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 Ivo Karlovic in the next round after the Croatian defeated countryman and 24th seed Ivan Ljubicic.

Kohlschreiber defeated two left-handers, Horacio Zeballos and Wayne Odesnik, en route to the third round and while he lost the opening two sets against Nadal he fully tested his more-fancied opponent.