Serena Williams withdrew in the night from Tuesday to Wednesday for the WTA tournament of Cincinnati. The 23-fold Grand Slam winner has too much trouble with her back.
The physical problems led to Williams giving up crying against Bianca Andreescu during the final of the WTA tournament in Toronto on Sunday.
"The tournament in Cincinnati is really one of my favorites. I did everything to get ready, but unfortunately my back is not in order," said the 37-year-old American.
Williams' decision comes just two weeks before the start of the US Open, where she is a six-time winner. Last year she had to acknowledge her superior in a much-discussed final in Naomi Osaka.
Williams, who needs one more Grand Slam title to match record holder Margaret Court, has been waiting for a tournament victory for more than 2.5 years. At the beginning of 2017 she booked her last success at the Australian Open.
The tournament in Cincinnati lost the title defender in the second round on Tuesday-evening, as Kiki Bertens went down in three sets against Williams's sister Venus Williams.