Kei Nishikori, glad to be back on the New York hard courts where he became Asia's first men's Grand Slam finalist, reached the second round of the US Open Tuesday.

The Japanese sixth seed, who fell to Marin Cilic in the 2014 final, quashed German Benjamin Becker's rally attempt, emerging with a 6-1, 6-1, 3-6, 6-3 victory on the new Grandstand court in two hours 10 minutes.

"It was a great feeling of course," Nishikori said of the lively reception he received from fans. "I have great memories here and I always love coming back to New York."

Nishikori appeared to be on his way to a quick victory when he put away the first two sets in 49 minutes.

Fifth-seeded Romanian Simona Halep survived late jitters to make it safely into the second round with a 6-0, 6-2 victory over Belgian Kirsten Flipkens.

Halep needed less than an hour, but admitted she was "too nervous to finish the match" after failing to take advantage of her first chance to close it out when leading 6-0, 5-0.

On Monday, defending champion Novak Djokovic overcame an arm injury scare and long-time rival Rafael Nadal shone in 33-degree heat as the US Open showcased its $650 million upgrade.

World number one Djokovic, also the 2011 winner in New York and chasing his third major of the year, defeated Jerzy Janowicz of Poland 6-3, 5-7, 6-2, 6-1.

Nadal, the 2010 and 2013 champion and seeded to face Djokovic in the semifinals, eased to a 6-1, 6-4, 6-2 win over Denis Istomin of Uzbekistan.

In the women's event, second seeded Angelique Kerber, the Australian Open champion, needed just 33 minutes to progress but Roland Garros winner Garbine Muguruza required three sets and a visit from the doctor before she survived.

Canadian fifth seed Milos Raonic, the runner-up to Andy Murray at Wimbledon, overcame German dangerman Dustin Brown 7-5, 6-4, 6-4.

Kerber needed just over 30 minutes to get her pursuit of a first US Open title off to a winning start.

The German was leading Polona Hercog 6-0, 1-0 when the Slovenian called it quits having won just nine points.

Kerber, a semifinalist in 2011, next faces Mirjana Lucic-Baroni of Croatia.

Muguruza carved out a gritty 2-6, 6-0, 6-3 win over Belgian qualifier Elise Mertens, ranked at 137.

But the French Open champion needed courtside treatment from a doctor after dropping the first set.

French 13th seed Richard Gasquet, a semi-finalist three years ago, was the day's biggest casualty.

He went down to 6-2, 6-2, 6-3 to British world number 84 Kyle Edmund.

Also exiting was Olympic champion Monica Puig with the Puerto Rican losing 6-4, 6-2 to China's Zheng Saisai.

Kayla Day, just 16 and ranked at a lowly 374 in the world, was 6-2, 4-2 up on Madison Brengle when her fellow American quit with an injury.

Day next faces American eighth seed Madison Keys who beat compatriot Alison Riske 4-6, 7-6 (7/5), 6-2 in a tie which ended at 1.48am, the latest finish to a women's match in tournament history.

Croatian seventh seed Marin Cilic, the 2014 champion and winner in Cincinnati last week, eased past Brazil's Rogerio Dutra Silva 6-4, 7-5, 6-1.

Flamboyant French 10th seed Gael Monfils took his summer hardcourt record to 15-2 with a 6-4, 6-2, 7-6 win over Gilles Muller of Luxemburg.