Sloane Stephens

The top seed appeared sluggish against Switzerland's Voegele, who claimed her biggest win in five years with a 6-4 5-7 6-2 victory.

Stephens started the quarter-final proceedings with errors on her backhand, giving Voegele the opportunity to break in the final game of the first set.

The American showed more aggression in the second, winning the last three games in a row to take the set, but she faltered in the decider as the Swiss dropped just two points in four games.

Spain's Muguruza suggested in the build-up that she was not fully fit and lost the first set 6-1 to unseeded Barbora Strycova of the Czech Republic.

Muguruza, the reigning Wimbledon champion, called a medical timeout at 2-0 down in the second set and then called it quits after having her pulse taken on the side of the court.

The 23-year-old said afterwards she had been too unwell even to practice for her opener but had been determined to play, having lost in the quarter-finals last week in Wuhan, where she struggled with a leg injury.

In the first All-American final since the Williams sisters played each other for the title in 2002, Stephens ran riot to win 6-3 6-0 in a surprisingly one-sided encounter on Arthur Ashe Stadium.

Stephens joins 2009 champion Kim Clijsters as the only unseeded women's champions in the Open era.

"It's incredible," said Stephens. "If someone told me when I had surgery that I would win the US Open, it's impossible I would say. Coming back, just being able to keep it all together, this journey's been incredible and I wouldn't change it for the world.