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.