NEW YORK (AP) -- The question was rather simple after Angelique Kerber became only the second defending U.S. Open champion in the professional era to lose in the first round.

The surprisingly lopsided 6-3, 6-1 loss to 45th-ranked Naomi Osaka of Japan under the closed roof in Arthur Ashe Stadium at a rainy Flushing Meadows on Tuesday was former No.

1, easily advanced by beating Magda Linette 6-2, 6-1, French Open champion Jelena Ostapenko got past Lara Arruabarrena 6-2, 1-6, 6-1 in a match that started on Court 17 then was moved indoors at Ashe, and No.

Kerber's exit further thins the women's bracket, which was missing 23-time major champion Serena Williams to begin with because she is expecting a baby.

She nearly upended Keys in the U.S. Open's third round last year, leading 5-1 in the third set before losing in a tiebreaker, a collapse on her mind in the latter stages Tuesday.