The American third seed seemed to struggle physically in high temperatures in New York, with the extreme heat policy again in place.
Stephens hit 27 unforced errors and lost 6-2 6-3 on Arthur Ashe Stadium.
Victory for Latvian Sevastova sees her reach the first Grand Slam semi-final of her career.
Stephens hit 27 unforced errors and lost 6-2 6-3 on Arthur Ashe Stadium.
Victory for Latvian Sevastova sees her reach the first Grand Slam semi-final of her career.
