Big-serving Isner will be tall order for Murray

At 6ft 3in there are not many players that Andy Murray looks up to. Tomorrow, however, the world No 4 faces another opponent from the valley of the tennis giants. Having disposed of the 6ft 8in Kevin Anderson earlier this week, Murray now faces all 6ft 9in of John Isner in the fourth round of the Australian Open.

Isner, who knocked Andy Roddick out of last year's US Open and is unbeaten this year after winning his first title in Auckland a fortnight ago, will present a formidable test. Of current players only Ivo Karlovic, at 6ft 10in, is taller than the 24-year-old American, who wears size 15 shoes.

Murray considers Isner's serve one of the best in the game. "He's a difficult guy to play against," Murray said. "He's the guy that everyone talks about all the time. You don't want Karlovic or Isner next to your name in the draw, because the match is on the other guy's racket. If he serves great and hits some big shots and returns, Isner is a really tough guy to beat. I'm going to have to be on my game.

"I've got a good record against Karlovic, and when I played Kevin Anderson, who's also a big guy, and Sam Querrey, I've always played well. But it does always come down to a few points, so it's really important to be on the ball when you get the chances, because they don't come so often."

While Murray maintained his record of not losing a set this year by beating France's Florent Serra 7-5, 6-1, 6-4 yesterday, Isner reached the last 16 of a Grand Slam tournament for the second time by beating Gaël Monfils, the world No 12, 6-1, 4-6, 7-6, 7-6. This is only Isner's third appearance at a Grand Slam event, the American having played tennis at university before making his bow on the professional tour.

Murray, who is two years younger than Isner, is a comparative veteran, with 16 previous Grand Slam tournaments under his belt. This is the seventh successive Grand Slam event in which he has reached the fourth round, though the world No 4 will be well aware of what happened on the two previous occasions he reached the last 16 here. While there was no shame in losing an epic five-set match to Rafael Nadal in 2007, his defeat to Fernando Verdasco 12 months ago was an unexpected setback, the Scot having started the tournament as one of the favourites.

As in both those previous runs to the fourth round here, Murray has yet to lose a set. Serra, the world No 64, rarely looked capable of denting that record from the moment Murray opened the match with three successive aces. Although Murray played an up-and-down first set, dropping his serve twice, some of his shot-making was breathtaking. In the middle of the match he was all but unplayable, winning 11 out of 12 games.

Nevertheless, Murray has yet to meet a player ranked in the world's top 50 and Isner could prove a major stumbling block, even though the American's fellow students used to call him "Grandpa" at the University of Georgia. "I'm so slow with everything I do," the world No 28 explained. "I don't get much accomplished. Off the tennis court, I just sit around, don't do anything."

On the court, however, Isner can be a fearsome proposition, particularly when serving. He hit 26 aces in beating Monfils, taking his total for the tournament to 81.

Isner said that holding serve and attacking at every opportunity would be his main aims. "Andy's such a really, really good player," he said. "I'm going to have to play really, really well to have a chance. He's one of the fittest guys on tour. He really can do anything. He can play offence. He can play defence. He can mix it up really well." The reward for the winner is likely to be a quarter-final against Nadal, who reached the fourth round by beating Philipp Kohlschreiber 6-4, 6-2, 2-6, 7-5. The defending champion plays Karlovic in the fourth round after the latter won a battle of the Croatian giants by beating Ivan Ljubicic in four sets.

Murray is the only Briton left in singles after Elena Baltacha (right) was beaten 6-1, 6-2 by Dinara Safina, the world No 2. The last time Safina played a Briton in a Grand Slam tournament she beat Anne Keothavong, Baltacha's predecessor as national No 1, 6-0, 6-0. While the Russian constantly had the world No 83 on the back foot with the power of her ground strokes, Baltacha did not look overawed on her first appearance in the Rod Laver Arena.

Baltacha said she would take confidence from her performances here. "I've had a fantastic start to 2010," she said. "I've had 12 matches already in three weeks. I want to build on this now. I want to get back and work on parts of my game and look forward."

Laura Robson, partnering the Australian Sally Peers, won again in the women's doubles, beating Taiwan's Chuang Chia-jung and the Czech Republic's Kveta Peschke 6-3, 6-4.