Madison Keys has withdrawn from the WTA Elite Trophy because of a knee injury.

Keys had been due to face Garbine Muguruza in the semi-finals in Zhuhai on Saturday, however, a problem with a left knee forced her withdrawal.

Wang Qiang will take the American's place in the last four, offering hope of home success in the tournament.

The injury ends another strong year for Keys, who reached the semi-finals of both the French Open and US Open, though major success still eludes her.

"I am very sorry to have to withdraw from the tournament," Keys said in a statement.

"Zhuhai is such a wonderful city and I love playing in front of the fans here, so this was a very difficult decision. Thank you for all of support and see you all in 2019."