A string of first-round upsets has thinned the women’s field at the US Open, with three of the tour’s most talked-about prospects—Canada’s Victoria Mboko, France’s Loïs Boisson and Britain’s Katie Boulter—departing Flushing Meadows within 48 hours. Mboko, seeded 22 after her breakthrough title in Montreal, fell 6-3, 6-2 to 29-year-old Barbora Krejcikova. The Czech, a two-time major champion and last year’s Wimbledon winner, broke serve in the opening game and needed just 83 minutes to advance, exploiting the teenager’s taped right wrist that has troubled her since her National Bank Open semifinal tumble. Boisson—world No.46 and a surprise semifinalist at Roland-Garros—could not convert a one-set lead against Switzerland’s Viktorija Golubic, losing 3-6, 7-6 (3), 6-2 after requiring medical attention for heat stress early in the decider. The defeat ends the 22-year-old Frenchwoman’s first appearance in New York and extends a run of early exits following her coaching split this summer. British hopes also took an early hit. Boulter, who will slide out of the top-50, double-faulted eight times in a 6-4, 6-4 loss to Ukraine’s Marta Kostyuk and later said she is considering outside help to fix persistent serving issues. Compatriot Sonay Kartal cramped badly in a three-set defeat to Brazil’s Beatriz Haddad Maia, 6-3, 1-6, 6-1, while fellow Britons Francesca Jones and Billy Harris were knocked out in straight sets. The early exits clear a path for higher seeds but reinforce the volatility of this season’s women’s tour. Krejcikova moves on to face Japan’s Moyuka Uchijima, while Kostyuk and Haddad Maia also progressed without dropping a set aside from the Brazilian’s brief physical struggle. Play continues Wednesday with defending champion Iga Swiatek and No.1 seed Aryna Sabalenka yet to open their campaigns.