Williams, 34, had last week accepted a wild card to defend her title after playing at the Olympic Games in Rio. Serena Williams could lose her world number one ranking this week after withdrawing from the Cincinnati Open because of a shoulder injury.
The American withdrew following a practice session on Monday. Williams has been ranked world number one for 183 consecutive weeks, and 306 in total.
She was knocked out of the Olympics in the third round by Ukrainian Elina Svitolina, and was beaten in the doubles with sister Venus in the first round – their first defeat in 16 Olympic matches together. The US Open, the final Grand Slam of the year, begins on 29 August.