When defending champion Naomi Osaka and teenage sensation Coco Gauff meet in the third round of the US Open on Saturday, even Serena Williams says she will be "excited to be a fan girl".

Gauff, 15, became a household name when she reached the fourth round at Wimbledon before losing to eventual champion Simona Halep.

It is only her second appearance in a Grand Slam singles main draw.

"I definitely think it's the future of women's tennis," said Williams.

"I think it's super-exciting tennis. It will be a really interesting match," Williams, who lost to Osaka in last year's final, added.

Japan's Osaka, who won the Australian Open in January, has struggled with form and injury since becoming world number one.

The 21-year-old reached just the third round at the French Open before being knocked out at the first stage at Wimbledon and has also failed to beat a top-10-ranked player since the start of the year.

Meanwhile, Gauff has continued her impressive form, winning her first WTA doubles title at the Washington Open alongside compatriot Catherine McNally, 17.

"She's done so well again here this tournament," said Swiss five-time champion Roger Federer. "I think we're all a bit surprised that she's able to back it up after Wimbledon which was already an incredible run.

"For her to strive in those conditions shows she's going to have a great future ahead of her. It's going to be a big day [on Saturday]. I wish her all the best, of course."

Seeds go head-to-head

Elsewhere in the women's draw, former Australian Open champion and two-time US Open finalist Caroline Wozniacki is up against Canadian 15th seed Bianca Andreescu on Arthur Ashe.

Nineteen-year-old Andreescu clinched her second WTA title in Toronto this month following Williams' retirement in the final and the teenager also beat German three-time Grand Slam champion Angelique Kerber in the final at Indian Wells in March.

Dutch seventh seed Kiki Bertens takes on German 26th seed Julia Gorges on Louis Armstrong, while Belgian 25th seed Elise Mertens faces Germany's Andrea Petkovic, ranked 88th, on Grandstand.

Elsewhere in the men's draw...

Spanish second seed Rafael Nadal takes on South Korean world number 170 Chung Hyeon in the second match on Arthur Ashe.

Nadal, who was knocked out by Argentine Juan Martin del Potro in last year's US Open semi-final, is seeking a fourth title in New York.

Australian 28th seed Nick Kyrgios, who lost to Nadal in the second round at Wimbledon, plays Russian Andrey Rublev, ranked 43rd.

And German sixth seed Alexander Zverev, who has failed to go beyond the quarter-finals of a Grand Slam this year, faces Slovenian world number 80 Aljaz Bedene on Louis Armstrong.