New Haven - Madison Keys withdrew from the Connecticut Open after injuring her neck on Sunday during a practice session ahead of the tournament she planned to use as a tune-up for the US Open.
Keys said she was hitting a forehand return when she felt pain on the right side of her neck. "I probably hit it late. ... As soon as I finished the stroke I knew that I did something," she said.
Keys, who is ranked No 9 in the world and seeded third in the tournament, has had trouble with her neck in the past.
She said she would seek medical treatment and hoped to be better in time for the US Open.
"Coming off a good Montreal (tournament) and having a good Olympics, I wanted to have another good week before the Open," she said.
The 21-year-old American reached the semi-finals in the Olympic women's tennis tournament in Rio de Janeiro, losing to Petra Kvitova of the Czech Republic in the bronze medal match.
Camila Giorgi of Italy replaced Keys in the Connecticut Open field as a lucky loser.
Ekaterina Makarova of Russia beat Sara Errani of Italy 7-5, 6-2 on Sunday; American Shelby Rogers topped France's Kristina Mladenovic 6-1, 6-1; and Elina Svitolina of the Ukraine edged Daria Kasatkina of Russia 6-4, 5-7, 6-0.
Top-seed Agnieszka Radwanska of Poland and second-seeded Roberta Vinci of Italy have byes to the second round, while defending champion Petra Kvitova and wild-card entrants Eugenie Bouchard of Canada and Caroline Wozniacki of Denmark are scheduled to play on Monday.
Czech players Barbora Strycova and Karolina Pliskova also withdrew.
Strycova pulled out with an illness and has been replaced by US qualifier Nicole Gibbs. Pliskova, the No 8 seed, withdrew with a lower back injury.
Her replacement in the field will be announced on Monday.