NEW YORK: The Serena Williams show continued at the US Open as the world number one moved a step closer to a calendar Grand Slam on Wednesday, while Mardy Fish took a final Flushing Meadows bow as the curtain came down on the American's career.

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."

FEW UPSETS

Two-time champion Nadal got his second round contest off to a sluggish start but the eighth seeded Spaniard was quickly in control, dispatching Diego Schwartzman 7-6(5) 6-3 7-5 as the Argentine saw his record against top 10 opponents drop to 0-6.

Djokovic, a five-time U.S. Open finalist but winning it all just once in 2011, has been in imperious form strolling into the third round with a clinical 6-4 6-1 6-2 win over Austrian Andreas Haider-Mauer.

After a first round littered with upsets, second round action went largely according to script as Spanish seventh seed David Ferrer beat Serb Filip Krajinovic 7-5 7-5 7-6(4) and Canadian 10th seed Milos Raonic fought off back pain to beat Spain's Fernando Verdasco 6-2 6-4 6-7(5) 7-6(1).

It was the end of road, however, for Fish, who announced earlier this year that he would retire after playing his 13th U.S. Open.

Following an 18-month layoff, Fish, who suffers from a severe anxiety disorder, returned to the ATP Tour in March and went down swinging in his Flushing Meadows farewell, stretching out his final match as long as he could before falling 2-6 6-3 1-6 7-5 6-3 to 18th seeded Feliciano Lopez.

"I have got a lot of great memories; I've got a lot of good wins out here," Fish said.

"I probably would have chosen this one as my last one regardless if I didn't have any issues with my health in the past couple of years, just because this is the biggest one and the most fun and the one that you want to go out on."