US Open champion Bianca Andreescu has withdrawn from the season-ending WTA Finals with a knee injury in China.

The Canadian teenager retired from her round-robin match against Karolina Pliskova on Wednesday with the problem.

Already unable to reach the semi-finals, Andreescu decided to pull out of the tournament after having a scan.

American alternate Sofia Kenin will instead face defending champion Elina Svitolina, who has already qualified for the last four, on Friday.

Andreescu is the second player to withdraw from the event in Shenzhen following world number three Naomi Osaka, who is in the other group.

The 19-year-old said she heard her knee "crack" and will now look to recover before the start of the 2020 season.

Andreescu will end the year inside the world's top six following an unusual breakout season that saw her claim a maiden Grand Slam title as well as encounter plenty of injury problems.

Ranked 152nd at the start of the season, she has won 48 of her 55 matches after completing just seven WTA tournaments.

As well as her stunning US Open win last month, Andreescu also lifted titles in Indian Wells and Montreal.

"I'm very disappointed to not be able to finish the year on my terms, but I am hopeful I will have more chances to play here in the future," said Andreescu, who played just two matches between the end of March and start of August because of a shoulder injury.