Dominic Thiem and Kristina Mladenovic sparkling form came as their relationship fell apart.

The tennis lovebirds appeared to be driving each other to new heights as Thiem made the final of the ATP's year-ending tournament in London and Mladenovic led France to Fed Cup glory in Australia.

But it's emerged the duo have split, ending a relationship that began in 2017, reports News.com.au.

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The concurrent successes of the pair became a storyline as Thiem defeated Roger Federer and Novak Djokovic before falling to Stefanos Tsitsipas in the ATP Finals.

Thiem's father, Wolfgang, was asked about his son's relationship with 26-year-old Mladenovic during the event and on reflection hinted at the shaky state of affairs when asked why she wasn't in London.

"They do not spend too much time together – even if there are combined tournaments that allow them to meet," he said.

"She has a lot to do with the French team for the Fed Cup in Paris. They celebrate the title. Very happy for her."

The largest newspaper in Thiem's homeland of Austria reported the break-up. "There have been rumours circling around London that he (Thiem) had split from his girlfriend Kiki Mladenovic even before the Masters run to the finals," it read. "He has hidden it out of respect for the newly crowned Fed Cup winner."

Thiem, 26, and Mladenovic started dating in 2017 after she asked him for a warm-up hit in practice.