MELBOURNE, Australia -- Defending champion Novak Djokovic reached the Australian Open quarterfinals for the fifth straight year with a 6-1, 6-3, 4-6, 6-3 win over former No. 1-ranked Lleyton Hewitt on Monday night.
"It's obviously the first match that I've been tested," the top-seeded Djokovic said. "It was against the player that I expected to be tested."
Djokovic is aiming to become only the fifth man in the Open era to win three consecutive majors after winning the Wimbledon and U.S. Open titles last year.
"Lleyton was playing in front of his crowd. Obviously, he loves competing against the top guys on the big stage, and he proved it again," Djokovic said.
Nobody had taken more than three games in a set off Djokovic in the opening three rounds. He was leading by two sets and a break before Hewitt, who was playing on a wild-card entry after his ranking plummeted in an injury-plagued 2011, ended that streak by winning six straight games to force a fourth set.
The loss for former U.S. Open and Wimbledon champion Hewitt ended his 16th Australian Open.
"I have to give credit to ... Lleyton, who never gives up," Djokovic said. "He's a great competitor and he obviously made me play an extra shot. I made a couple of unforced errors and I got him back to the match. He's very well known for that."
Two-time Australian Open runner-up Andy Murray only spent 49 minutes on court and was leading 6-1, 6-1, 1-0 when Mikhail Kukushkin retired from their match with a left hip injury, giving him an easy path into the quarterfinals.
"It's obviously good for me, I get to conserve some energy," Murray said. "Tough for him, first time in the fourth round of a Slam."
The 22-year-old Nishikori became the first Japanese man to advance to the Australian Open quarterfinals in 80 years, and only the second man from his country to reach a Grand Slam quarterfinal since the Open Era started in 1968. Shuzo Matsuoka reached the 1995 Wimbledon quarterfinals.
Roger Federer more or less held a clinic at Rod Laver Arena, where he has won four of his 16 Grand Slam titles, on Sunday night -- a 6-4, 6-2, 6-2 victory over Bernard Tomic that moved him into the quarterfinals for a 31st consecutive major.
Federer's quarterfinal will be his 1,000th tour-level match. He plays 2009 U.S. Open champion Juan Martin del Potro, whom he once expected to rise to No. 1.
Rafael Nadal advanced with a convincing win over fellow Spaniard Feliciano Lopez. The 2009 champion's right knee was heavily wrapped and his left ankle needed to be taped after three games of the first set. Afterward, he said he was fine.
He will play Tomas Berdych, hoping to avoid a third consecutive quarterfinal loss in Melbourne.
Information from The Associated Press was used in this report.