Four-time champion Caroline Wozniacki is out of the Connecticut Open, having lost 7-5 6-2 to Jelena Ostapenko on Monday (Tuesday AEST) in the first round of the final warmup for the US Open.
Wozniacki, now ranked world No.51, was coming off a second-round loss to Petra Kvitova at the Rio 2016 Olympic Games.
The 39th-ranked Ostapenko will face top-seeded Agnieszka Radwanska in the second round.
Elena Vesnina of Russia and Eugenie Bouchard of Canada won their opening matches. Bouchard rolled past Annika Beck of Germany 6-2 6-1 and Vesnina beat Camila Giorgi of Italy 6-1 6-4.
The 20th-ranked Vesnina arrived in New Haven on the heels of a doubles gold medal at the Rio Olympics with Ekaterina Makarova. She reached her first major singles semifinal at Wimbledon.
In other matches on Monday, Anastasija Sevastova beat Nicole Gibbs of the US 7-5 6-0 and Caroline Garcia of France completed a rain-delayed 6-1 6-4 victory over Irina-Camelia Begu of Romania.