Sloane Stephens was knocked out of the U.S. Open in the second round. (AP Photo/Kathy Willens)

Sloane Stephens, the third-highest-ranked American women’s tennis player, stumbled out of the U.S. Open in the second round on Wednesday.

The 21-year-old — and No. 21 seed at the event — lost to Sweden’s Johanna Larsson in three sets, 5-7, 6-4, 6-2, to mark her earliest exit from the tournament’s main draw. No. 4 seed Agnieska Radwanska of Poland also lost to China’s Shuai Peng, 6-3, 6-4.

Stephens’s loss comes a day after 15-year-old American CiCi Bellis had a surprise of her own when she upset No. 12 seed Dominika Cibulkova in the first round to become the youngest player to win a U.S. Open match since Anna Kournikova in 1996.

Stephens, ranked No. 24 in the world, appeared sluggish at times and committed a whopping 63 unforced errors. Larsson, the 96th-ranked player in the world, had never advanced past the first round at the U.S. Open in her previous four attempts.

Stephens has struggled this year compared with her breakout season in 2013, when she reached the semifinals at the Australian Open and the Wimbledon quarterfinals. Stephens failed to advance past the fourth round in any of the majors this year.

The American appeared defensive in her news conference after the match, per the Post’s Isabelle Khurshudyan.

Scoreboard: ATP | WTA