Defending men's champion Marin Cilic and two-time winner Rafa Nadal and world number one Novak Djokovic also eased into the third round with straight sets wins as the temperatures and stakes continue to rise at the year's final grand slam.

Bidding to become just the fourth woman to win all four slams in the same year, no one has more riding on the Flushing Meadows fortnight than Williams, who continued her quest with an error-strewn 7-6(5) 6-3 victory over Dutchwoman Kiki Bertens.

The 110th-ranked Bertens put up a valiant fight and had Williams, who struggled with her serve firing 10 double faults, on the ropes in the opening set.

However, the 33-year-old American's class shone through in the second set as she broke Bertens three-times to seal her 23rd consecutive U.S. Open victory. "I never stop. I keep going and give the best effort that I can," Williams said after raising her 2015 match record to 50-2.

Williams was joined in the third round by sister Venus who outlasted Irina Falconi 6-3 6-7(2) 6-2 to set up an intriguing clash with Swiss teenager Belinda Bencic, one of only two players to beat Serena this season.

Born in 1997, the same year Venus made her U.S. Open debut, Bencic survived a second set meltdown that left her sobbing and a determined Misaki Doi of Japan to claim a nervy 5-7 7-6(3) 6-3 victory. "I think we all have not-so-good days," said Bencic. "I know I shouldn't have behaved like that."

After playing his opening match on an outside court, Cilic took his U.S. Open title defence back to a sweltering Arthur Ashe Stadium and looked right at home, storming into the third round with a 6-2 6-3 7-5 win over Russian qualifier Evgeny Donskoy.

During last year's magical run Cilic was invincible on the main court, sweeping past Tomas Berdych in the quarters, taking out Roger Federer in three sets in the semi-finals and Kei Nishikori in a one-sided final. "This court is very special for me, brings back special memories," the ninth seeded Cilic said. "It's sort of unreal. I was working all my life to win a grand slam and now I'm in position to defend one."

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