Updated

By Greg Stutchbury

MELBOURNE (Reuters) - Andy Murray supporters in the Hisense Arena momentarily held their breath on Friday when the Briton stopped and sunk to his knees after being twisted the wrong way by a Florent Serra winner.

The 22-year-old, seeded five at the Australian Open, had immediately grasped at his lower back and his face wore an expression of anguished pain.

The world number four, however, recovered and went on to overcome the crafty Frenchman 7-5 6-1 6-4 and advance to the fourth round, where he will meet big-serving American John Isner.

More importantly, Murray said afterwards there was no problem with his back after the twinge.

Murray said after his win on Friday he was slowly building into the tournament with his serve, return and ability to win the big points, which he may need to improve his 2-7 record against Nadal.

"I'm happy to have won in three sets in all of the matches," Murray said. "It's important to have tight situations in matches like the first set today where it got quite close at points.

(But I) came through all the sort of tough moments in the match.

"Everyone is panicking about my serve (but) I'm happy with how it's gone. It's got better in each match, like I thought it would.

"When the important moments have come, I've served well. That's the most important thing."

(Editing by Patrick Johnston)