Melbourne: Rafael Nadal on Saturday said only a mental block was stopping Andy Murray from becoming the world's best player after he blew a golden chance to reach the Australian Open final.

Nadal, who will play Novak Djokovic in Sunday's title match, said Murray could be crowned world number one this year and was "unlucky" not to have won a grand slam.

But he highlighted a weak fourth set in Friday's epic, near five-hour clash with Djokovic, when Murray threw away a two-sets-to-one lead. The Scot fought back in a nail-biting fifth but he cracked when serving to stay in the match.

Spain's Nadal, a 10-time grand slam winner, said it was an opportunity missed for Murray, who is seeking to end Britain's 76-year wait for a major men's title.

"Yesterday he lost another very good opportunity, in my opinion," Nadal said. "To win a tournament like this and to play against a player like Djokovic, you cannot start the fourth set like this.

"It's the moment to play with more intensity than ever, not start with 3-0 down and two breaks in five minutes. That way you lose the match.

"You want to win the tournament, the other can beat you, but you cannot lose in the beginning."

Murray took pride in his performance, after his straight-sets hammering by Djokovic in last year's final, and said he was closing the gap on the top three, who also include Roger Federer.

But Nadal said fourth-ranked Murray was "never very far" from the leading trio, who have annexed the major titles and the top ranking in recent years.

"He's playing well. He's playing fantastic. He deserves to win a grand slam," Nadal said. "He was unlucky at the end, but the only negative thing for me in his match is the beginning of the fourth.