The Biebers served up some major star power at the US Open Friday night … prompting a shoutout from tennis great Coco Gauff -- and paving her way to victory.
Justin and Hailey were kicking back in their box seats at Arthur Ashe Stadium, watching Coco battle Elise Mertens in the third round of the women's grand slam tournament.
Hailey looked like her beautiful self in a trendy leather coat … but Justin stole the show with his funky pink glasses and polka-dot black cap turned back around.
The couple was clearly there to support Coco, jumping to their feet and cheering whenever she won points at critical stages in the match.
Coco went on to beat Mertens in three sets -- 3-6 6-3 6-0.
The 19-year-old phenom gave the Biebers credit for inspiring her to win by their very presence after she dropped the first set.
Coco told an ESPN reporter, "I definitely saw who was there. I thought, 'I cannot lose in front of Justin Bieber.' I didn't lose a game after I saw that."
She added, "I was definitely starstruck with Justin Bieber. And then Hailey coming, too. I guess it was a little date night for them which is really cool.
By the way ... Coco's favorite Bieber songs are "Never Say Never" and "Baby."