Washington, DC, USA, 21 July 2016 | tennis.com.au

Sam Stosur has moved into the quarterfinals of the Citi Open in Washington DC after opponent Caroline Wozniacki withdrew from their match in the second set.

Stosur was trailing 5-7 4-3 before the former world No.1 pulled the pin, citing a left elbow injury.

“She started kind of running around her backhand a little bit more, which is pretty unusual for Caroline to want to do that. So I kind of thought, ‘OK, there’s something going on’,” Stosur said.

“Sometimes it’s quite difficult playing somebody who’s injured.

“(But) it was a really tough match and I thought we were playing some really good tennis.”

Earlier in the match, Stosur had led 5-3 and served for the match at 5-4, only for Wozniacki to reel off a string of unanswered games to pocket the first set.

Stosur broke serve in the opening game of the second set, yet handed the break back straight back when she dropped serve to love.

But with Wozniacki increasingly struggling on her two-handed backhand, it was proving difficult for her to compete.

The Dane held a break point in the sixth game for a 4-2 lead, but Stosur erased it with a forehand winner and in the next game broke Wozniacki’s serve to take control of the second set.

Stosur, the top seed at the International-level WTA tournament, goes on to face American wildcard Jessica Pegula.

“I feel good. I’m happy to be in the quarters now and I’ll take each match as it comes. I’m really looking forward to the next one and enjoying every day here,” Stosur said.