WASHINGTON (AP) — Japan's Risa Ozaki upset second-seeded Sloane Stephens in straight sets on the women's side, and eight-seeded Sam Querrey advanced on the men's side Tuesday night at the Citi Open.

Stephens, the defending champion, lost 6-2, 6-1 to Ozaki in her opening match. Querrey, coming off a trip to the Wimbledon quarterfinals that featured a victory against world No. 1 Novak Djokovic, beat Bjorn Fratangelo 6-7 (5), 6-4, 6-4 on center court.

Stephens said it was just one of those matches for her and took all the blame for her defeat. The 23-year-old American said she'd be upset for 10 minutes and then get over it.

"I just played real bad. Nothing more, nothing less," Stephens said. "Just one of those days I was really bad. Obviously I'm not the first person to have a bad day and I won't be the last."

Querrey battled back from losing a first-set tiebreak to beat Fratangelo under the lights at Rock Creek Park Tennis Center. He'll next face 11th-seeded Alexandr Dolgopolov of Ukraine.

"It was one of those matches I just had to fight my way through," Querrey said.

Earlier in the day, fifth-seeded Eugenie Bouchard was upset by Italy's Camila Giorgi, 7-5, 6-4, marking the fourth time in the Canadian's past 10 tournaments that she lost her opening match. She 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 Tuesday, when temperatures were in the 90s, but she 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 repeated that she was unsure about playing in the Rio Olympics, citing the Zika virus. The Montreal native called herself a "last-minute person" and said she would wake up and decide next week.

Bouchard has seen fellow Canadian Milos Raonic and other high-profile tennis players and golfers pull out and conceded that has made her think.

"I'm truly undecided, and you see more and more people pulling out as kind of the deadline gets closer," Bouchard said. "It's just unfortunate because it would be my first games, and to have a problem like this kind of dampening the excitement of potentially your first Olympic Games, it really sucks, to be honest."

Querrey already announced he's skipping the Olympics but not because of Zika.

"It just wasn't a priority of mine at all," said Querrey, who played in the 2008 Olympics in Beijing.

In other men's play, 12th-seeded Grigor Dimitrov lost 6-4, 6-4 to Great Britain's Daniel Evans. On the women's side, 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.

McHale continued a strong year that has included pushing world No. 1 Serena Williams to three sets twice.

"I'm putting myself more in matches to have opportunities to win," McHale said. "I want to stop meeting (Williams) so early in tournaments. It's nice to see against the No. 1 player in the world that I can hold my own and have chances."