The No. 2 seed won in straight sets; Agassi and Federer to meet in the quarters.
MELBOURNE, Australia (AP) -- Andy Roddick overcame some frustrating moments before advancing to the Australian Open quarterfinals today with 6-3, 7-6 (8), 6-1 win over German qualifier Philipp Kohlschreiber.
The second-ranked Roddick lost just three points on his serve in the first set, but was more erratic in the second, doubling his number of unforced errors to 12 and throwing his racket to the court after sending a forehand long.
After taking the tiebreaker on consecutive errors by the 102nd-ranked Kohlschreiber, Roddick dominated the third set, breaking the German twice.
At times, Roddick lacked intensity against the lowest-ranked man remaining in the draw, relying on his big serve for motivation. He served 15 aces, many just when games were looking stale.
In what's shaping as an Australian Open classic, top-ranked and defending champion Roger Federer and four-time winner Andre Agassi will meet in the quarterfinals.
Agassi weathered a record 51 aces by Sweden's Joachim Johansson to win 6-7 (4), 7-6 (5), 7-6 (3) 6-4 and advance.
Johansson beat Richard Krajicek's tour record of 49 aces, set in a quarterfinal loss to Yevgeny Kafelnikov at the 1999 U.S. Open.
Federer, who is on a career-best, 25-match winning streak, had a victory that was far less complicated than Agassi's -- a 6-2, 6-2, 7-6 (4) win over Cypriot qualifier Marcos Baghdatis.
Federer, whose 11 titles in 2004 included three majors, has won 48 of 50 matches. The defending champion hasn't lost since the second round at the Athens Olympics and has a four-match winning streak against Agassi.
Marat Safin, who lost to Federer in last year's Australian final, converted just one of his 18 break-point chances in a fiery 4-6, 7-6 (1), 7-6 (5), 7-6 (2) win over Olivier Rochus.
Two of Russia's three major champions advanced Sunday to the quarterfinals -- U.S. Open champion Svetlana Kuznetsova and Wimbledon champion Maria Sharapova.
Kuznetsova beat one Russian -- 6-4, 6-2 over Vera Douchevina -- and faces another in the quarters. Sharapova, seeded fourth, will face Kuznetsova after rallying past Italy's Silvia Farina Elia 4-6, 6-1, 6-2.
Seventh-seeded Serena Williams beat No. 11 Nadia Petrova 6-1, 3-6, 6-3. Second-seeded Amelie Mauresmo topped Evgenia Linetskaya 6-2, 6-4 in a mistake-prone match with 11 service breaks.