Defending champion and former world number one Victoria Azarenka has withdrawn from the Western and Southern Open with a right knee injury.
The two-time Australian Open champion, who only made her return from a foot problem at the Aegon International in June, had been due to play Barbora Zahlavova-Strycova in the first round of the WTA Premier event in Cincinnati on Tuesday.
Azarenka beat Serena Williams in three sets to take the 2013 showpiece, but will not get a chance to defend that crown.
Despite the setback, the Belarusian is confident that she will be fit for the US Open - which gets under way in 13 days' time.
"It's tough to withdraw as defending champion," Azarenka said. "[The Western and Southern Open] is a great tournament and I really enjoy it.
"I should be good for the US Open."
Since her return to action, Azarenka has failed to add to her tally of 17 WTA singles titles and was knocked out in the second round at Wimbledon by Bojana Jovanovski.
The 25-year-old has reached the final of the US Open at Flushing Meadows in each of the last two years, losing to world number one Serena on both occasions.
Zahlavova-Strycova will now face Germany's Mona Barthel, who takes Azarenka's place in the draw as a lucky loser from qualifying.