It was a painful day for Bianca Andreescu ©Getty Images

Karolina Pliskova got off the mark at the Women's Tennis Association (WTA) Finals in Shenzhen, China, and only had to play one set. 

The number two seed from the Czech Republic took the first set against number four seed Bianca Andreescu 6-3, who was then forced to retire.

There was no indication of any injury problems when the Canadian took the first two games of the match. 

However, Andreescu pulled up after stretching to return a serve. 

She continued after getting her left knee heavily strapped, but it was obviously affecting her game. 

"I didn't know it was really that bad," Pliskova said. 

"Of course I feel sorry for her… I just wanted to get this victory to keep myself in the tournament, which happened, of course not in the best way." 

A tearful Andreescu cannot now qualify from the purple group, while Pliskova will face Simona Halep to decide who makes it through to the semi-finals.

Elina Svitolina is the first player through to the last four ©Getty Images
Elina Svitolina is the first player through to the last four ©Getty Images

Halep is now in the last-chance saloon, having lost to eighth seed Elina Svitolina from Ukraine.

Defending champion Svitolina became the first player to secure a place in the last four, with one group game still to play. 

She was too strong for Halep, coming home in straight sets 7-5, 6-3. 

Svitolina will top the purple group if she wins her final match against Andreescu, providing she recovers from her injury.

The WTA Finals event features the world's top eight players competing in two round-robin groups of four, with the winners and runners-up advancing to the semi-finals.