Caroline Wozniacki breezes into US Open semis

Sevastova needed heavy strapping on her right ankle after falling awkwardly in only the second game. Dane Wozniacki, who is unseeded in New York after slipping to 74th in the world, will play German second seed Angelique Kerber in the last four. Kerber, 29, defeated Italy's Roberta Vinci, the 2015 finalist, 7-5 6-0.

The decisive moment of the second women's quarter-final came as early as the second point of game two, when Sevastova moved to her right and tumbled over while trying to make a service return.

By the time the trainer returned at 6-0 3-0, she had won just 15 points. It took 52 minutes for Sevestova, who beat Garbine Muguruza and Johanna Konta to reach the last eight, to finally win her first game, but the outcome was never in doubt.

"I tried my best," said Sevastova. "I don't like to retire, but Caroline played great tennis. Even if I had no injury, it would be tough to beat her."

Wozniacki said she felt sorry for her opponent but was looking ahead to the rest of the tournament.

"I always believe in myself, no matter what my ranking," said the former world number one. "I've beaten pretty much everyone in the draw before." Kerber can overtake Serena Williams as world number one this week and is also on course for a third Grand Slam final in 2016.

To remain at the top of the rankings, Williams, who plays Simona Halep later on Tuesday, must reach the final again. If she plays Kerber in the final, the American would have to win.

Vinci caused one of the greatest upsets in tennis history when she beat Williams in last year's semi-final to end her calendar Grand Slam hopes, but a foot fault on set point all but ended her challenge against Kerber.

The Italian, 33, broke Kerber's serve three times in the first set but each time was broken back. She was then called for a foot fault facing set point with a second serve. She sarcastically applauded the line judge, before Kerber raced through the second set in 24 minutes.