World number two Roger Federer may have felt he was a little off-key yesterday, but he did enough to beat the in-form David Nalbandian in four sets and seal an Australian Open semi-final showdown with Juan Carlos Ferrero.

The Spanish third seed had earlier wrestled past spirited Moroccan Hicham Arazi 6-1 7-6 (8-6) 7-6 (7-5) to reach his first Australian Open semi-final.

On the line tomorrow will not only be a place in the final of the season-opening Grand Slam, against either Andre Agassi or Marat Safin, but the world number one ranking.

Victory over the French Open champion will ensure Federer replaces the deposed Andy Roddick and tops the rankings for the first time his career.

And after doing just enough to quell Nalbandian, who had breezed into the last eight without dropping a set, he must be favourite.

Federer's gliding rhythm that had swept Lleyton Hewitt aside was missing under spitting Melbourne skies, but in the way of a true champion he played the crucial points with a confident artistry no other player possesses.

Despite 55 unforced errors, Federer completed a 7-5 4-6 7-5 6-3 victory over the Argentinian who had, up until November last year, held sway over him.

At the Masters Cup in Houston, Federer beat Nalbandian for the first time in six meetings and defeated Ferrero and beat Agassi to win the title. It was an experience that has left him brimming with confidence.

''Now I play Hewitt, to play Nalbandian to play Ferrero. That is tough, especially over five sets, every second day, but I am looking forward to it, I'm confident now,'' he said.

''Even though I played a better match against Hewitt I feel I have definitely got the game to beat Ferrero.''

Federer did not play badly but his execution was not as lethal as it can be. But as Nalbandian conceded afterwards, it was his ability to step up a gear which proved the difference.

''It was so close,'' said the former Wimbledon finalist.

''I think in the important points, in the big points, he played better than me. That's the difference.''

Federer faced two break points when serving at 5-5 in a tight first set of few openings. His response was emphatic: four successive aces to take the game and force the pressure back on Nalbandian.

The Argentinian immediately lost his serve, Federer was a set up and on his way.

''I've never hit four aces in a row. That was very important at that stage. They came at the right time. That was maybe the key of the match. Who knows?'' said Federer.

The pattern was set. Nalbandian broke twice in the second set but Federer replied in kind each time and then took it 6-4.

There was no lack of fight from Nalbandian who pressured the Federer serve with great hustle and sealed the third set to keep himself in the match.

But Federer always appeared composed and broke Nalbandian at the first opportunity and served out the win in four sets. The win left him delighted, the performance less so.

''It was not that great a match, it didn't live up to my expectations,,'' he said. ''I couldn't find my rhythm. I took more chances than he did. My game is always to attack so I'm going to hit unforced errors. I agree, I hit too many today."