Roger Federer, far short of his fluent best, beat Spain's Tommy Robredo 6-3, 7-6, 7-5 on Tuesday to set up an Australian Open semi-final showdown with Andy Roddick.

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On the other hand, Roddick played almost flawless tennis to demolish fellow American Mardy Fish 6-2, 6-2, 6-2 earlier in the day.

The top seed and defending champion struggled in blustery conditions against the seventh seed, but still did enough at key moments to extend his winning streak to 34 matches and stay on course for his 10th Grand Slam title.

"Conditions were very slow and it was windy so I had to adjust to that," Federer told a news conference. "Tommy is a tough baseliner so he plays it smart. I tried to change things up a bit but I struggled to find my rhythm.

"Overall, I played pretty well. The breaks of serve were due to the windy conditions. My attacking style worked out well and I volleyed really well. As long as I get through I'm happy."

Roddick battered his former high school friend in just one hour and 27 minutes, making only four unforced errors to advance to the last four at Melbourne Park for the third time.

"I played pretty flawless," Roddick told a news conference. "It's probably the best I've played against him. I think Mardy might have been a little bit nervous at the beginning as he made a few errors he wouldn't normally make. But that's about as well as I can play."

Roddick blasted through the first set in 29 minutes and the unseeded Fish, who upset fourth seed Ivan Ljubicic in the first round, simply did not possess the attacking weapons to disrupt the rhythm of his Davis Cup team mate.

The 25-year-old Fish, appearing in his first Grand Slam quarter-final, broke back to 1-1 at the start of the second set but Roddick simply lifted his game, thundering down 10 aces and a blistering range of groundstrokes to overwhelm his opponent.

Federer started slowly on Rod Laver Arena, making a number of uncharacteristic errors, but the Swiss number one still broke Robredo's serve three times in the opening set.

The Spaniard took advantage of this carelessness to contrive two breaks of his own but Federer typically seized on his first set point with a flowing backhand down the line.

The world number one continued to misfire in the second, mixing a couple of wild mis-hits with a succession of weak groundstrokes into the net, and Robredo stayed with him until the top seed broke for 5-4.

Federer, 25, then played a poor service game, slipping to 15-40 before sending a tame forehand volley wide.

Robredo comfortably held serve but Federer raised his game in the tiebreak, roaring into a 6-2 lead before clinching it 7-2.

Federer broke early in the third set but handed the advantage straight back and if the 24-year-old Robredo had been able to push harder he could have extended the Swiss maestro further.

Robredo had to serve at 5-6 to save the match and Federer pounced, wasting his first match point with an errant backhand down the line before forcing the Spaniard to drift a backhand wide to seal victory in two hours 20 minutes.

It was the fourth time Federer has reached the semi-final of a grand slam without dropping a set and his match against sixth seed Roddick will be his 11th successive appearance in a Slam semi-final, beating Ivan Lendl's professional era record of 10 in a row.

Federer has won the Australian Open twice, beating Marcos Baghdatis in last year's final and Marat Safin in 2004.

"I felt great going into the game physically and I feel like I'm hitting the ball well," Roddick said. "The gap on Roger has probably been closing a little bit and I'm playing about as well as I can."

Roddick beat Federer 6-2, 3-6, 6-3 in the Kooyong Classic final prior to the Australian Open.