With pounding rain creating a din under the new USD 150 million roof of the Arthur Ashe stadium, Murray battled through a tense first set to triumph 6-4 6-1 6-4 here yesterday.
"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.
Both Serena and Venus Williams looked set to sample play under the roof for the first time.
Serena, who matched the Open Era record for Grand Slam singles titles with her 22nd at Wimbledon, said she'd be aiming to step it up a notch against Vania King in the second round last night.
"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.