MELBOURNE, Australia (AP) Defending champions and top-ranked Serena Williams and Novak Djokovic had matches starting at the same time on nearby courts.

Six-time champion Serena Williams raced to a 6-1, 6-1 win in 45 minutes over 18-year-old Russian Daria Kasatkina on Rod Laver Arena. She will next play Margarita Gasparyan, who beat Yulia Putintseva 6-3, 6-4.

Djokovic took 25 minutes to win the first set against No. 28-seeded Andreas Seppi but needed almost an hour in the next two, saving two set points in the tiebreaker before winning 6-1, 7-5, 7-6 (6) on Margaret Court Arena. After making way for Williams on the main court, he noted it was his first match on Melbourne Park's No. 2 court since it was renovated to include a roof.

Djokovic has won five Australian titles and his victory maintained a streak of reaching at least the fourth round at every major since the 2009 French Open. He was also particularly wary of a third-rounder against Seppi, who beat four-time champion Federer at the same stage here last year.

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Seppi ''is not overwhelmed by the occasion of the big courts, he knows how to play the big stage. He beat Federer last year here in the Australian Open, so he knows how to play big matches,'' Djokovic said. ''He's very dangerous. I'm just glad to get through in straight sets.''