Kiki Bertens admitted she was quite content sitting on the couch before making a stunning start to the WTA Finals when she was called in as a last-minute alternate.

The Dutchwoman replaced Naomi Osaka after the Japanese injured her shoulder and swiftly made an impact in the Red Group, defeating world number one Ashleigh Barty.

It was a fine display from Bertens in Shenzhen as she battled from a set down to triumph 3-6 6-3 6-4, securing a first win over Barty at the sixth attempt. 

"I was okay sitting on the couch," she said. "I was really tired. But then yesterday I just hit for five or 10 minutes. I just rested completely. I think today I felt already much better.

"Of course, it's not like after one day that you're completely back in full energy. As the match was continuing, I felt better and better.

"The movement was getting a bit better. I was starting to feel the court. I could play a little bit more aggressive, come to the net."