No. 2 Chanelle Scheepers and fifth-seeded Barbora Zahlavova Strycova both lost first-round matches at the Citi Open on Monday.
Scheepers, ranked No. 42 in the world, fell 6-2, 6-1 to Magdalena Rybarikova. Zahlavova Strycova was beaten by Edina Gallovits-Hall, 7-5, 4-6, 6-3.
No. 7 seed Coco Vandeweghe avoided an upset by defeating Olga Govortsova 6-4, 6-1. Eighth-seeded Olga Govortsova topped Karolina Pliskova 6-2, 6-2, and Leonardo Mayer beat Guillermo Garcia-Lopez 7-6 (5), 7-6 (8) to advance to the second round.
The Washington tournament, a staple on the ATP schedule, is combining with the WTA Tour for the first time.
Mardy Fish, the top-seeded men's player, is scheduled for his opening match on Tuesday. Fish, the No. 13 ranked player, skipped the Olympics after missing several weeks on tour earlier this year following a procedure to correct a heart problem.
Russian Anastasia Pavlyuchenkova is the top seed in the women's draw.
Scheepers committed five double faults and won only 48 percent of the points on her first serve. That paled in comparison to Rybarikova, the 102nd ranked player this week.
Rybarikova won 20 of 23 points on her first serve, had the only five aces in the match, and jumped out to a 5-0 second-set lead.