Karolina Pliskova reached the first Grand Slam final of her career at last year's US Open, beating both Williams sisters en route to the final (Venus in the quarterfinal, Serena in the semifinal). She eventually lost to Angelique Kerber in the final.
The current World No.1 faces American qualifier Nicole Gibbs today - and holds a 5-0 record against players from the US so far this year, beating Asia Muhammad in Brisbane, Madison Brengle in Miami, CoCo Vandeweghe in Stuttgart, Lauren Davis in Rome, and Alison Riske in Eastbourne.
Fourth seed Elina Svitolina will be confident facing Evgeniya Rodina; she has a 2-0 record against her, with both matches on hard courts, and both finishing in straight sets.
Kurumi Nara is looking for the first Top 10 win of her career as she faces Svetlana Kuznetsova. They have played each other four times before, with the Russian winning every time, and dropping only one set.
Kuznetsova can sympathize with Angelique Kerber. The Russian lost to Ekaterina Bychkova in the first round of the 2005 US Open as she was bidding to defend her title.
French Open champion Jelena Ostapenko faces World No.54 Sorana Cirstea today. Seven of the Latvian's 15 defeats this year have come against players ranked outside the Top 50: Lauren Davis in Auckland (No.61), Donna Vekic in St Petersburg (No.88), Wang Qiang in Dubai (No.76), Madison Brengle in Miami (No.86), Kristyna Pliskova in Prague (No.58), Varvara Lepchenko in Toronto (No.68) and Aleksandra Krunic in Cincinnati (No.85).
Madison Keys is currently the third-ranked American, behind the Williams sisters - and is one of ten players from the host nation to reach the second round at Flushing Meadows this year.
Ons Jabeur became the second Tunisian woman to win a Grand Slam main draw match when she beat Ana Bogdan 6-3, 6-4 at Roland Garros this year.
Before the start of the tournament, Shelby Rogers had not won a match on a hard court so far this summer, losing in the first round in Washington (to Mariana Duque Marino) and in qualifying in New Haven (to Aliaksandra Sasnovich).
Nicole Gibbs talks to Ed McGrogan at tennis.com about how she is coping with a difficult year.
Keep up with the twists and turns in the Battle for No.1 - WTA Insider has all the latest.
Maria Sharapova looked impressive as she fought back from a set down to beat Timea Babos. Catch up with the highlights.