No.4 seed Hsieh Su-Wei is the only remaining seeded player in the draw, and will bid for her first semifinal in North America against 17-year-old wildcard Catherine McNally - who is playing her first WTA quarterfinal in her third main draw at this level. McNally has notched up five Top 100 wins - all this year, with the best by ranking being World No.54 Alison Van Uytvanck in the Surbiton ITF W100 quarterfinals - but Hsieh will be the first Top 50 player the American has gone up against.
Lauren Davis is the only player remaining in the draw who has reached the final in Washington - and the 2016 runner-up faces a rematch of her semifinal that year against compatriot Jessica Pegula, which Davis won 6-2, 6-3. The head-to-head between the Americans is all square at 2-2, with Pegula having taken both encounters since that Washington semifinal - 6-4, 2-6, 6-2 in Québec City qualifying last year (going on to reach her maiden WTA final) and 6-3, 6-4 in the Newport Beach 125K semifinals this February.
Today will mark the first time Kristina Mladenovic and qualifier Anna Kalinskaya have played outside Moscow, where they have already squared off in two first-round encounters - both won by Mladenovic, 6-4, 6-2 in 2016 (Kalinskaya's WTA main draw debut) and 6-1, 6-3 in 2018.
The head-to-head between Zarina Diyas and Camila Giorgi is tied at 1-1, with both previous matches having gone the distance. Diyas was a 7-6(4), 3-6, 6-1 victor in Miami qualifying in 2014, while Giorgi won their 2016 Hobart first-rounder 6-4, 1-6, 6-1.
Anna Kalinskaya is bidding to become the first qualifier to reach the last four in Washington. The Russian is the third qualifier to reach the quarterfinals here in nine years of the tournament being held, following Jana Cepelova in 2012 and Allie Kiick in 2018.
ORDER OF PLAY
JOHN HARRIS COURT (starts 2pm)
 Maria SANCHEZ (USA) / Fanny STOLLAR (HUN) vs  WANG Yafan (CHN) / YANG Zhaoxuan (CHN)
After 2 ATP matches
[WC] Coco GAUFF (USA) / Catherine MCNALLY (USA) vs  Anna KALINSKAYA (RUS) / Miyu KATO (JPN)
GRANDSTAND (starts 1pm)
Jessica PEGULA (USA) vs Lauren DAVIS (USA)
[Q] Anna KALINSKAYA (RUS) vs Kristina MLADENOVIC (FRA)
 HSIEH Su-Wei (TPE) vs [WC] Catherine MCNALLY (USA)
Camila GIORGI (ITA) vs Zarina DIYAS (KAZ)