Exuberant Russian Marat Safin announced his return as a major force in astonishing fashion with a thrilling five-set victory over world No 1 Andy Roddick to set up an enticing Australian Open semi-final with Andre Agassi.

It was a performance which showed in all its glory the 'New Marat' - the player's own way of referring to the mature version of himself, as opposed to the playboy model he was in 2002, when he lost the final when red-hot favourite.

This year there is no bevy of blondes in his camp. Safin is here with his mother and two coaches, determined to regenerate a career which stalled last year when he was forced to spend difficult months out with a wrist injury.

It proved a blessing in disguise. He went camping in Yosemite National Park, spent relaxing days fishing and drinking wine - and as his wrist recovered so his love for tennis returned.

The former world No 1, currently ranked 86 after six successive first-round defeats last year, was leaden-footed in the first set.

But he took a painkiller for a sore groin, and suddenly the sparks began to fly.

Safin, unseeded, won 2-6 6-3 7-5 6-7 (0/7) 6-4, while Agassi strolled through when Sebastien Grosjean retired with a groin strain.

''I enjoy my life so much; maybe I enjoy it too much, so much that I start to miss tennis,'' he said.

''I did a little bit of fishing, a little bit of camping; all these kind of things make you think about life and what you need to do to improve your tennis.

''I had a small cut in the ligament and I had problems with the cartilage and nerve. It took me one month and a half. I had nothing to do, so I went camping.

''I was a bit depressed. It was really good just to think a little bit, no people.''

There are elements of the 'Old Marat' still evident in his game. The birthday boy - 24 yesterday - still wears his heart on his sleeve. But when things were tough against Roddick he dug in resolutely.

The top seed had been in blistering form and carried that into the first set to break three times. And when Safin began to feel his groin it appeared all was lost.

The pair's vastly different routes through to the last eight seemed to be telling.

Roddick had not dropped a set, and his fourth-round victory over Taylor Dent, in which he conceded just four games, was the easiest of his Grand Slam career.

Safin, meanwhile, was averaging almost three hours a match.

He began sluggishly but he was determined to fight and emerged from that injury time-out a changed man.

Safin was effective at the net and unleashed a number of punishing groundstrokes to break Roddick and level the piece at one set all.

By now his injury concerns seemed forgotten as he produced a third set of wonderful all-court tennis, full of clever volleys, blistering passing shots and subtle lobs which even had Roddick applauding.

The fourth set was tighter. There were few points against the serve, and Roddick twice had to save the set and force a tie-break, which he won 7-0.

''I felt I was lucky to win that fourth set,'' said Roddick.

''I thought he was playing better that set. I felt like I had more chances to win in the fifth. He just took his chance when he got it. That was the main difference.''

Roddick had the momentum but could not break when Safin went into his shell for the first few games of the deciding set, and it proved costly when he was broken for a 5-4 lead.

The American earned two break-back points, but Safin saved both on his way to a remarkable victory.

Scores: Men's Singles Quarter Finals

(4) Andre Agassi (USA) bt (9) Sebastien Grosjean (Fra) 6-2 2-0 ret, Marat Safin (Rus) bt (1) Andy Roddick (USA) 2-6 6-3 7-5 6-7 (0-7) 6-4

Women's Singles Quarter Finals

(32) Fabiola Zuluaga (Col) bt (4) Amelie Mauresmo (Fra) w/o, Justine Henin-Hardenne (Bel) bt (5) Lindsay Davenport (USA) 7-5 6-3