Defeat for Karolina Pliskova on Saturday would bring to an end her chances of retaining her World No.1 spot. Garbiñe Muguruza and Elina Svitolina are standing by to take advantage if the Czech makes an exit.
Karolina Pliskova has won all four previous meetings with Shuai Zhang, with her last three victories over the Chinese player all coming in straight sets. Their most recent meeting was in Doha earlier this year, when Pliskova prevailed 6-2, 6-0 in their quarterfinal match-up.
Shelby Rogers seeks a first career victory over Elina Svitolina, having lost their previous two matches - in Osaka in 2014 and in Brisbane earlier this year. Both went to three sets after Rogers won the second.
Jelena Ostapenko hopes to avenge her most recent meeting with Daria Kasatkina, losing in Charleston earlier this year. The Latvian prevailed in their two earlier matches, winning in Trnava in 2015 and again in Eastbourne last year.
CoCo Vandeweghe's sole victory in her six matches with Agnieszka Radwanska came in Birmingham last summer, the American winning a three-set match in 136 minutes. The Pole has not dropped a set in any of their other meetings.
Steve Tignor gives his take on Maria Sharapova's reaching the second week at Flushing Meadows for Tennis.com.
Petra Kvitova put on a superb display to overwhelm Caroline Garcia, as Paul Newman of The Independent examines.
Ava Williams of the Washington Post takes a look at Sloane Stephens' chances of reaching the quarterfinals at the US Open for the first time.
Three players are in contention to end the tournament as World No.1. Check out all the explanations and permutations here.
Catch up on Saturday's Order of Play here.
Highlights of Maria Sharapova's victory over Sofia Kenin: