With the field at Flushing Meadows now whittled down to 16, wtatennis.com and SAP thought it time to go looking for answers...
320 - Keys has spent 320 minutes on court thus far - the most of any remaining player.
85 - The percentage of first-serve points won by Serena Williams in her opening three matches - the best among the 16 players left in the draw.
31 - Serena has unsurprisingly hit more aces, 31, than anyone else en route to the fourth round. Following closely behind is Keys with 25.
18 - Konjuh, 18, is the youngest player left in the draw. It is the third year in succession a teenager has reached the fourth round.
12 - Kvitova has dropped fewer games, 12, than any other player en route to the fourth round.
7 - Seven of the Top 10 on the Road To Singapore leaderboard are still in contention for the US Open title: Serena, Kerber, Agnieszka Radwanska, Carla Suárez Navarro, Keys, Simona Halep and Pliskova. The only absentees are Dominika Cibulkova and Garbiñe Muguruza.
5 - An unseeded player has reached the quarterfinals at Flushing Meadows on each of the past three years. Konjuh, Anastasija Sevastova, Yaroslava Shvedova, Tsurenko and Caroline Wozniacki are all bidding to keep this run going.
3 - Three players - Konjuh, Karolina Pliskova and Tsurenko - are through to the last 16 at a Grand Slam for the first time
1 - For the last eight years, at least one Italian has reached the US Open quarterfinals. Roberta Vinci, the 2015 runner-up, looks to make it nine when she takes on Tsurenko. The Americans have been even more dominant: 1993 was the last time a home player failed to make the last eight (Lindsay Davenport and Navratilova fell in the last 16).
0 - The number of times Shvedova has beaten Serena in four career meetings. She did famously come within a couple of games of doing so four years ago at Wimbledon.