TENNIS/Australian Open Championships:The general view, since Roger Federer missed the pre-Australian Open warm-up exhibition in Kooyong with a stomach illness, is that the great man is vulnerable.

Every one of his matches has been minutely analysed and every time he has deviated from perfection teeth have been sucked and I-told-you-so glances exchanged. Yet here he is in the semi-finals again for the 15th consecutive slam with Serbia's 20-year-old Novak Djokovic, the world number three, standing between the Swiss and the chance to win his 13th major. And this may be the biggest test of all.

The rise of Djokovic has been remarkable. A year ago he was pushing and straining for a place in the top 10 and drawing plaudits wherever and whenever he played. Now he has accelerated away in terms of achievement and experience. His 6-0, 6-3, 7-5 quarter-final victory over Spain's David Ferrer, the world number five, saw him reach his fourth consecutive slam semi-final.

Djokovic is remarkably mature and has impressed everybody on and off the court. His post-match mimicry of other players, has caused something of a stir inside the locker room. For the most part he has let his tennis do the talking and he has achieved a level of controlled excellence, combined with an unbending mental fortitude, that has lifted him to an undisputed third place in the world behind Federer and Rafael Nadal.

Tomorrow evening in the Rod Laver arena is the chance for him to step up on to a more elevated plane. The last player to defeat Federer in a slam, other than Nadal on the clay of Roland Garros, was Russia's Marat Safin in the semi-finals of this tournament three years ago. Djokovic recorded his first and only victory against the Swiss in the final of last year's Canadian Tennis Masters Cup. For him to repeat that here is a colossal ask, though not beyond him.

"It's difficult to compare this year with last. I was told that the year after your breakthrough year is the most difficult but I'm feeling really, really good, both physically and mentally," he said.

There seems little doubt that Federer will regard this latest challenge with a degree of circumspection. At Djokovic's first major final at the US Open last year when Federer beat him 7-6, 7-6, 6-4 the Serb was within a shot or two of making serious inroads and his stature has hardly diminished since then, even though he had a horrid time in the end-of-season Tennis Masters Cup. Much hard work throughout December has restored his batteries.

"I've been playing really consistently here and I think I have improved. I've played with more patience, and have also been more and more aggressive."

He has taken inspiration from the fact that his fellow Serb Janko Tipsarevic took Federer to 10-8 in the fifth set on Saturday.

Federer, shifted through the gears against James Blake and accelerated away under the starry skies for a 7-5, 7-6, 6-4 quarter-final victory of his own. The American had never beaten him in seven attempts, managing just a single set in the process, so there was always an air of the inevitable about this match. There was plenty of absorbing tennis, with magnesium flashes of blinding brilliance, but only ever one outcome.

As usual Federer will concentrate on his own game rather than worrying about Djokovic: "I go a lot with my feelings throughout a match. If I need to adjust something I'll do it - either playing more aggressively or more defensive. I used to concentrate much more on my opponent when I was younger but right now it's automatic. I don't have to change my game plan a whole lot except for Rafa because he's a lefty."

Five matches, five wins, the last two in straight sets. If this is Federer being vulnerable then the word has to be given a new meaning.

"I look forward to playing my best tennis against Novak. I look forward to playing to win. That's my mindset."

Nobody ever really doubted it, though Djokovic must.