Italy's Camila Giorgi upset fifth-seed Eugenie Bouchard 7-5, 6-4 on Tuesday in the first round of the Citi Open.
It's the fourth time in the Canadian's past 10 tournaments that she has lost her opening match. Bouchard most recently reached the round of 32 at Wimbledon before losing to Dominika Cibulkova.
Bouchard said she felt kind of out of it from the beginning but put no blame on the heat and instead credited Giorgi for keeping her off balance.
"She kind of plays not like any other player, really: absolutely no rhythm, not long points at all," Bouchard said. "I found it hard to get my feeling and get into points and stuff."
Bouchard battled back from a 4-1 first-set deficit to tie it at 4- and 5-all before Giorgi held serve and broke her to win 7-5.
When Bouchard was broken to fall behind 4-2 in the second set, she threw her racket to the ground. After battling back to get it to 5-4, she was broken again to lose the match.
Bouchard said afterward she was still unsure about playing in the Rio Olympics, citing concerns about the Zika virus. The Montreal native said she was "a last-minute person" and would wake up and decide next week.
She may stay in Washington rather than going home early for the Rogers Cup, even though she was eliminated earlier than she expected.
The 24-year-old Giorgi will face Austria's Tamira Paszek in the second round.
"I think everything was fine," Giorgi said of her game. "I was really solid in the baseline and the return of serve was aggressive. Everything was going forward my game I play, so it went good."
In other women's play, American Christina McHale upset eighth-seeded Monica Niculescu of Romania 6-3, 6-4. The New Jersey native will next face Jessica Pegula, daughter of Buffalo Bills and Sabres owners Terry and Kim Pegula.
On the men's side, eighth-seeded Sam Querrey was set to face Bjorn Fratangelo in night action.