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Serena Williams could be set for her stiffest challenge so far at this year's US Open when she faces fellow American and 2017 champion Sloane Stephens for a place in the last 16.

The pair have played each other six times, with 38-year-old Williams coming out on top on five occasions, including at the 2013 US Open, which she won.

However, this is their first meeting since the 2015 French Open.

"She's a great competitor," said the 23-time Grand Slam singles winner.

"You can't win a Slam and not be really, really, really, really good."

Third seed Williams, seeking to equal Margaret Court's record of 24 Grand Slam singles titles, added: "It's always going to be intense. Always going to be, you know, who I am on the court, so that's the only thing I can do.

"And the only difference is I'm playing such a good player so early, so I have to bring what I can even more."

Saturday's match is scheduled to be second on the Arthur Ashe Stadium.

Bulgarian Tsvetana Pironkova will look to topple another top-ranked player when she faces Croat 18th seed Donna Vekic. The 32-year-old has already caused a stir in New York having seen off two-time major winner Garbine Muguruza in the last round.

Her feats so far have been all the more remarkable because this is her first event since Wimbledon 2017, after which she gave birth to her first child.

Former world number one Victoria Azarenka, coming off the back of winning her first title since 2016, faces Poland's Iga Swiatek while her doubles partner and Australian Open winner Sofia Kenin, is up against dangerous 27th seed Ons Jabeur of Tunisia.

Seventh seed Madison Keys, who Stephens defeated in the final three years ago, will play France's Alize Cornet in the first match of the night session on Ashe (00:00 BST).

In the men's competition, it promises to be a battle of the big serves as Austrian second seed Dominic Thiem comes up against 2014 champion Marin Cilic of Croatia after Keys-Cornet.

"It's not a guy I want to face in the third round," said Thiem, a three-time Grand Slam finalist.

American interest in the men's singles will be focused on Frances Tiafoe and JJ Wolf. Tiafoe, who beat Australia's John Millman on Thursday, is up against Hungary's Marton Fucsovics, while Wolf opens up proceedings on Ashe at 17:00 BST against third seed Daniil Medvedev of Russia.

Canadian Felix Auger-Aliassime, the 15th seed who overwhelmed Andy Murray, will face France's Corentin Moutet, the victor over another Briton, Dan Evans.

Vasek Pospisil, will be hoping for another big scalp following his surprise win over fellow Canadian and 24th seed Milos Raonic. The world number 94 faces Spanish eighth seed Roberto Bautista Agut.

Jamie Murray and Neal Skupski play their second-round men's doubles match against Marcelo Demoliner and Matwe Middelkoop at 16:00.