Novak Djokovic and Caroline Wozniacki reach the semi-finals of the US Open in New York

Defending US Open champion Novak Djokovic reached the semi-finals at Flushing Meadows after a knee injury forced Jo-Wilfried Tsonga to quit when trailing 6-3 6-2. The Serb's latest success means he has now reached the last-four stage of the tournament despite having only completed two of five matches in New York, having benefited from a walkover and two retirements.

Djokovic, 29, arrived at the US Open in the midst of a poor run of form, but having taken some time off following his exploits at the Rio Olympics, he now feels close to his best.

"I got a lot of days off and recovered my body," Djokovic said, according to the BBC. "Right now I'm feeling very close to the peak. That's the position where I want to be."

Djokovic will now face Frenchman Gael Monfils in the semi-finals after he beat compatriot Lucas Pouille 6-4 6-3 6-3. The 30-year-old star has yet to drop a set in five matches at this year's US Open.

Elsewhere in New York, Caroline Wozniacki progressed to a fifth US Open semi-final after overcoming Anastasija Sevastova 6-0 6-2. The Latvian was hampered by injury during the match and Wozniacki, 26, made light work of her opponent.

"I always believe in myself, no matter what my ranking," Wozniacki – who has slipped to 74th in the world – subsequently said, according to the BBC. "I've beaten pretty much everyone in the draw before."

The unseeded Dane will play German second seed Angelique Kerber in the last four after she defeated Italy's Roberta Vinci 7-5 6-0.