World number two Andy Murray made himself heard at the US Open on Thursday, downing tenacious Spaniard Marcel Granollers in straight sets to reach the third round.

With pounding rain creating a din under the new $150 million roof of the Arthur Ashe stadium, Murray battled through a tense first set to triumph 6-4, 6-1, 6-4.

"We're lucky that we get to play under the roof because otherwise there'd be no tennis," said Murray, whose match went on as all action stopped on the outside courts. "It's good for everyone."

But in the 22,000-seat stadium, the acoustics of the roof are proving problematic, and Murray admitted the echoing noise made things difficult.

"It was tough, you couldn't really hear the ball at all," said Murray, who is trying to become just the fourth man to reach all four Grand Slam finals in the same year.

His consistency in the majors paid off with a second Wimbledon title in July, a victory followed by a second straight Olympic gold in Rio.

He looked on his way to a routine victory over 45th-ranked Granollers with a 4-1 lead in the opening set, but wasted two set points in dropping his serve in the ninth game before finally breaking Granollers in the next on his seventh set point.

"Thankfully I got through that 5-4 game and then the momentum was back with me," said Murray, who broke Granollers twice in the second set and once in a tightly contested third to seal the win.

As the showers lingered, 11 doubles matches scheduled for outside courts were cancelled and some remaining matches reassigned.

The night session on Ashe remained as scheduled, with Serena Williams, who matched the Open Era record for Grand Slam singles titles with her 22nd at Wimbledon, aiming to step it up a notch against Vania King.

"I think I can get a lot better," the US superstar said after a straight-sets win over Ekaterina Makarova, although she acknowledged that she didn't know how the troublesome right shoulder that has hindered her since Wimbledon would react to her workout against the Russian lefty.

Even if it does flare up, Williams has to be considered a strong favorite against her 87th-ranked compatriot King, a wild card she trounced 6-1, 6-0 in their only prior meeting, in the second round of the 2014 US Open.

Juan Martin del Potro, who has undergone four wrist surgeries since lifting the US Open trophy in 2009, continues his latest comeback bid with a second-round clash against 19th-seeded American Steve Johnson.

Venus Williams, the sixth seed whose seven Grand Slam titles include two US Opens, coasted to the 70th US Open match win of her career 6-2, 6-3 win over Germany's Julia Goerges.

The elder Williams, bedevilled by 63 unforced errors in a scrappy first-round win over Ukrainian Kateryna Kozlova, cut that to 17 in a crisp performance against 64th-ranked German Goerges.

Fired-up fifth seed Simona Halep also advanced, downing Czech Lucie Safarova 6-3, 6-4 in a battle of former French Open finalists.

Halep called playing under the Ashe roof "amazing," although it didn't relieve all climate problems.

"I was a little cold," she said. "I felt the air conditioning."

Among those who didn't have the luxury of the roof, 2014 finalist Kei Nishikori of Japan waited out a third-set rain delay en route to a 6-4, 4-6, 6-4, 6-3 victory over 20-year-old Russian qualifier Karen Khachanov.

He was to be followed on the Louis Armstrong court by women's fourth seed Agnieszka Radwanska and men's third seed Stan Wawrinka.