The iron grip Roger Federer has held over the Australian Open tennis championship has come within a handful of points of being prised loose by Russia's Nikolay Davydenko.

But the world No.1 and top seed survived in four sets, scrambling into a semi-final against Germany's Nicolas Kiefer.

For Federer, the 6-4 3-6 7-6 (9-7) 7-6 (7-5) quarter-final win proved an ordeal.

For his future opponents it proved he was fallible.

"We always have tough matches," Federer said.

"This was one of the toughest. I'm very happy."

While the top-seeded Swiss appeared to play at a level below his best, he said that was only because Davydenko, the fifth seed, had performed above himself.

"I played at my best, I couldn't win if I didn't," he said.

The quarter final began as expected.

The top seed closed it out in 36 minutes and appeared to be on his untroubled way to another semi-final.

But Davydenko served notice early in the second set that he would fight it out, which he did for more than three hours.

He held two break points in the third game, but failed to convert.

Offered the same opportunity on Federer's next serve, Davydenko grabbed it to take a 3-2 lead, breaking again in the ninth game level the match.

With Federer's touch, and perhaps his concentration, slipping early in the third, Davydenko took advantage, breaking serve to lead 5-3, only to drop serve himself in the next game.

Federer struggled on most service games and when he stumbled into a hole at 5-6 and 15-40, it appeared to be a deep one.

A full-stretch backhand down the line saved the game, but it didn't stop Davydenko who was back again in the tiebreaker.

With a 4-1 lead he had the set in his grasp, and by the time he'd set up his fourth set point for the game it seemed he had to nail his man.

But Federer saved them all, finally securing the set 9-7 in the tiebreaker when Davydenko double-faulted.

Any relief that may have resulted from his survival will have lasted only a couple of minutes with Davydenko beginning the fourth by breaking Federer's opening serve.

But Federer broke back immediately, just as he had been in the previous set.

As the match progressed the standard of play retreated, but always the feeling was that Federer would find his right form.

He did in the vital tie-breaks, but only just.

Kiefer endured an even more testing marathon of almost five hours before defeating Sebastien Grosjean of France.

He meets Federer in a semi-final on Friday.

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