Rafael Nadal admits he must step up a gear to beat Andy Murray after blunting the power of giant Croatian Ivo Karlovic yesterday to set up an enticing Australian Open quarterfinal against the Scot.

The defending champion tamed the 6ft 10in (2.08m) Karlovic despite the Croat thumping down 28 aces, eventually winning 6-4, 4-6, 6-4, 6-4.

Nadal, a six time Grand Slam winner, now faces fifth seed Murray, with the in-form Briton yet to drop a set in the tournament.

The Spaniard has won seven of their nine previous matches, but said that record would count for little here and he would need to improve on his form against Karlovic to reach the semis.

"He is one of the more difficult players to play against," Nadal said of Murray, who beat another big-server, American John Isner, 7-6 (4), 6-3, 6-2.

"He can play aggressively and he can play defensively, he can do a lot of different things during a match.

"I have to play better next match if I really want to have chances to win."

Murray (pictured) is shouldering mounting expectations as he bids to become the first British man to win a Grand Slam singles title since 1936, but Nadal said he had no doubt the 22-year-old would win majors before his career was over.

"He has enough results to be confident of winning a Grand Slam," he said.

"He is going to have a lot of chances to win a Grand Slam, and he is going do it for sure."

The young Scot showcased his exciting strokeplay against Isner and believes he has the game plan to overcome Nadal.

"I played some really good matches against him on hard courts," said Murray, who lost to Nadal in five sets in the fourth round of the 2007 Australian Open.

"I think I've got some tactics that work well against him.

"I'm playing well. No question about that. I just need to play like I have been and maybe a bit more if I want to win the tournament."

Nadal had no qualms about facing Murray in the quarters, as opposed to deeper in the tournament.

"The important thing is I am in the quarterfinals, losing only two sets," he said.

Coming off injury problems that blighted his 2009, Nadal said he was pleased to be in such good form at the start of the new year.

"Being in the quarterfinals is a good result for me," he said. "It gives me a lot of confidence."

Despite the 39th-ranked Karlovic's booming serve, Nadal made it three wins from as many meetings with the Croat by claiming three crucial service breaks in the match.

Boosted by his swag of aces, Karlovic actually hit 55 winners, while Nadal managed 35.

However, Nadal's return consistency was the difference and he conceded few points cheaply, making just 15 unforced errors, compared to the Croatian's 42.