Simona Halep has withdrawn from the Connecticut Open in New Haven, citing a left leg complaint.
Halep rose to the number two ranking after making finals at the Rogers Cup and the Western and Southern Open, losing the deciders to Belinda Bencic and Serena Williams respectively.
The Romanian said playing another week in the lead up to the US Open, starting on August 31, was not viable.
"I am very sorry that I have to withdraw from New Haven," a statement from Halep read.
"I love the tournament and obviously have great memories there having won the title before.
"I have had a tough few weeks that have left me with pain in my left leg and unfortunately I can't force myself to play so close to the US Open."
Halep was replaced in the draw at the WTA Premier event by lucky loser Lesia Tsurenko, who will take on Barbora Strycova for a spot in the quarter-finals.
She will take the second seeding behind Grand Slam bidder Serena Williams, when the draw for the Flushing Meadows major takes place on Friday.