According to a report in today's The Sun, the former X Factor judge has been quietly dating the Bulgarian tennis professional after meeting recently.
A source said: "They're completely besotted with each other. But they've been careful not to be pictured together because they don't want the pressure of the relationship being in the public eye."
Nicole previously dated Formula One star Lewis Hamilton, while Grigor's last known girlfriend was fellow tennis star Maria Sharapova. He also dated Serena Williams.
During an interview with ITV's This Morning earlier this month, Nicole said that she was single, adding of love: "It's not where my focus is. I've just been so busy with everything."