Anna Kalinskaya upset Stephens in straight sets.
Russian qualifier Anna Kalinskaya defeated Sloane Stephens in straight sets (6-3, 6-4) on Tuesday, ending the American's hopes of another U.S. Open title.
Stephens, the 2017 tournament champion, struggled early, dropping the first set after committing 16 unforced errors to Kalinskaya's six.
Stephens took a 2-1 lead in the second set before Kalinskaya fought her way back to win 6-4. The 20-year-old fired 18 winners to 15 unforced errors en route to her first-round victory.
Since kicking off her summer hard-court campaign, Stephens, 26, has won just one match in three tournaments, falling in her Washington and Toronto openers. In Cincinnati, she needed three sets to defeat Yulia Putintseva before losing to eventual finalist Svetlana Kuznetsova.
Kalinskaya's win on Tuesday marked her first-ever Grand Slam victory. She will face Kristie Ahn in the second round on Thursday.