Washington - Former world number one Caroline Wozniacki suffered another setback in season of struggles Wednesday, retiring from a match she led against Australian top seed Samantha Stosur with a left arm injury.
"It's one thing after another," Wozniacki said. "After going out in the first round at Wimbledon I'm hitting the ball better and things are going well.
"It's really disappointing."
Wozniacki said she would need an exam to determine her next move and her status for future events, notably the Rio Olympics, which opens August 5.
Wozniacki, selected as Denmark's flagbearer, had to win an appeal to the International Tennis Federation to secure a place in Rio, the ITF saying she had not played enough Fed Cup matches to qualify for Rio. Her homeland's tennis federation successfully argued the ankle injury limited her ability to play in the event.
At 58th, she is out of the world top 50 for the first time since 2008.
Two-time US Open runner-up Wozniacki struggled last year with an ankle injury and in 2014 after a breakup with former fiancée Rory McIlroy.
Stosur, the 2011 US Open champion, was battling with Wozniacki in the first set when disaster struck the Dane.
"It happened at 5-all in the first set," Wozniacki said. "I hit a backhand and I just felt the pain."
The soreness was on the inside of her arm opposite from her elbow, but she kept playing and took the first set, Stosur unaware of Wozniacki's troubles.
"I could hit a forehand and run a lot of balls down," Wozniacki said. "She kept hitting to my forehand. As soon as I got a backhand there was a rush of pain.
"At that point, there was nothing I could do."
Stosur advanced to a quarter-final against US wildcard Jessica Pegula.
"I'll take it and try to run with it as far as I can," Stosur said.
The Aussie said she "couldn't see anything" but began to notice odd behaviour from Wozniacki.
"She started running around her backhand more than usual, which I thought was kind of odd for Caroline," Stosur said.
"Sometimes it's hard to play somebody who is injured. I had to stick with what I thought was working at that point."
Results on Wednesday from the third day of the ATP and WTA Washington Open (x denotes seed):
Steve Johnson (USA x5) bt Adrian Mannarino (FRA) 6-3, 6-4
Gilles Muller (LUX x14) bt Yoshihito Nishioka (JPN) 6-4, 6-1
Alexander Zverev (GER x7) bt Taylor Fritz (USA) 6-4, 6-2
Ryan Harrison (USA) bt Viktor Troicki (SRB x10) 7-6 (7/4), 6-4
Gael Monfils (FRA x2) bt Lu Yen-hsun (TPE) 6-3, 6-2
Ivo Karlovic (CRO x13) bt Brian Baker (USA) 6-3, 7-6 (7/4)
Malek Jaziri (TUN) bt Kevin Anderson (RSA x9) 5-7, 6-4, 7-6 (7/3)
John Isner (USA x1) bt James Duckworth (AUS) 6-3, 6-4
Marcos Baghdatis (CYP x15) bt John Millman (AUS) 6-2, 6-4
Jack Sock (USA x6) bt Lukas Lacko (SVK) 7-6 (7/1), 6-4
Borna Coric (CRO x16) bt Yuichi Sugita (JPN) 6-4, 6-4
Bernard Tomic (AUS x3) bt Donald Young (USA) 7-6 (7/4), 6-3
Yanina Wickmayer (BEL x7) bt Zhang Shuai (CHN) 6-3, 7-5
Jessica Pegula (USA) bt Christina McHale (USA) 7-5, 6-2
Samantha Stosur (AUS x1) bt Caroline Wozniacki (DEN) 5-7, 4-3, retired, left arm
Yulia Putintseva (KAZ x6) bt Usue Arconada (USA) 6-4, 4-6, 6-4
Lauren Davis (USA) bt Shelby Rogers (USA) 5-7, 7-5, 6-3
Kristina Mladenovic (FRA x4) bt Samantha Crawford (USA) 6-2, 2-6, 6-3