Serena Williams could lose her world number one ranking this week after withdrawing from the Cincinnati Open because of a shoulder injury.
Second seed Angelique Kerber will replace Williams at the top of the standings if she wins the tournament.
Williams, 34, had last week accepted a wild card to defend her title after playing at the Olympic Games in Rio.
"My shoulder inflammation continues to be a challenge, but I am anxious to return as soon as possible," she said.
The American withdrew following a practice session on Monday.
Williams has been ranked world number one for 183 consecutive weeks, and 306 in total.