'I can't lose in front of Justin Bieber!' Star-struck Gauff fights back

Coco Gauff was left star-struck by Justin Bieber in the crowd during her US Open match with Elise Mertens, but the 19-year-old American said the singer's presence also inspired her comeback victory on Friday.

Gauff dropped the opening set at Arthur Ashe Stadium but rallied, sealing a 3-6 6-3 6-0 win and reaching the fourth round with Bieber and his wife, Hailey, watching.

"I definitely saw who was there," Gauff said on the ESPN broadcast after the match. "I thought, 'I cannot lose in front of Justin Bieber'. I didn't lose a game after I saw that.

"I got a little tight when I first saw him, then I remembered President (Barack) Obama and Michelle Obama were at my first-round match."

Sixth seed Gauff, who is chasing her first Grand Slam title, said she was honoured to have celebrities in the stands during her matches this year.

Apart from the Obamas and the Biebers, the likes of Miami Heat six-time NBA All-Star Jimmy Butler and actress Katie Holmes have also attended her matches.

"It's crazy. I've come to the Open for years. Being a fan, just looking in the crowd to see who's coming, looking in the president's box, the suites, to see who's there, now being a player, it's really crazy," Gauff said.

"I was definitely star-struck with Justin. 'Never Say Never' was like the first, I think one of my favourite songs when I was a kid. Then 'Baby, Oh, Baby'. I could go through all his songs.

"Then Hailey, too, coming. I guess it was a little date night for them, which was really cool."

Gauff, who faces former world No. 1 Caroline Wozniacki next, said there was another celebrity who would leave her star-struck.

"Definitely Beyonce," Gauff said. "If she ever came, oh my goodness, I hope they wouldn't show her on the screen. Maybe at the end."