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.

Dominic Thiem (AUT x8) bt Ricardas Berankis (LTU) 6-4, 6-3, 6-2

Pablo Carreño-Busta (ESP) bt Janko Tipsarevic (SRB) 3-6, 4-6, 6-1, 6-4, 6-4

Nick Kyrgios (AUS x14) bt Horacio Zeballos (ARG) 7-5, 6-4, 6-4

Kei Nishikori (JPN x6) bt Karen Khachanov (RUS) 6-4, 4-6, 6-4, 6-3

Nicolas Mahut (FRA) bt Paul-Henri Mathieu (FRA) 6-4, 6-4, 6-2

Jared Donaldson (USA) bt Viktor Troicki (SRB) 7-5, 6-3, 6-3

João Sousa (POR) bt Feliciano Lopez (ESP x16) 6-2, 6-4, 1-6, 7-5

Illya Marchenko (UKR) bt Damir Dzumhur (BIH) 6-2, 6-4, 6-3

Grigor Dimitrov (BUL x22) bt Jérémy Chardy (FRA) 4-6, 6-4, 3-6, 6-4, 6-2