Caroline Wozniacki entered the Connecticut Open as a wild card, hoping a tournament she has won four times would help her tune up for the US Open.
Instead, the former top-ranked tennis player from Denmark saw her struggles continue in a 7-5 6-2 first-round loss to Jelena Ostapenko of Latvia on Monday (Tuesday AEST).
"It's been a really weird year for me, something I'm not used to," said Wozniacki, who has dealt with injuries all season.
"It's frustrating when you practice well and can't really execute in the matches ... At least when I know when I'm playing my best level, if someone beats me, that's fine. But what's frustrating is when you're not playing your best and then you get beat."
Wozniacki, now ranked world No.51, was coming off a second-round loss to Petra Kvitova of the Czech Republic at the Rio 2016 Olympic Games.
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Kvitova advanced on Monday night with a 1-6 6-1 6-3 win over American Louisa Chirico. She will next face Eugenie Bouchard, a 6-2 6-1 winner over Annika Beck of Germany.
World No.20 Elena Vesnina also advanced, with a 6-1 6-4 win over Camila Giorgi of Italy.
Top-seeded Agnieszka Radwanska was to have met Wozniacki in the second round. Instead, Radwanska, ranked No.5, will face Ostapenko, 19, who reached the final at Doha in February and won the Wimbledon junior girls tournament in 2014.
Against Wozniacki, Ostapenko staved off a set point in the first set, won four straight games to close out the set and never looked back.
"After that, the match turned the other way," Ostapenko said of her service breaks. "She's a great player ... so I just played like I had nothing to lose."
Wozniacki's early exit leaves the tournament with four players in the WTA's Top 20 - Radwanska of Poland, Vesnina of Russia, Kvitova of the Czech Republic and second-seeded Roberta Vinci of Italy.
In other matches on Monday, Anastasija Sevastova of Latvia, Caroline Garcia of France, Evgeniya Rodina of Russia, Annet Kontaveit of Estonia and Ana Konjuh of Croatia all advanced to the second round.
