"It's no secret that Rihanna and Pharrell have chemistry," the Mirror quoted a source as saying.
"And not just in the studio - they seem mesmerised by each other. But Chris will be gutted to learn he could have lost Rihanna to one of the sexiest men in music. Rihanna and Pharrell talk on the phone all the time and whenever she can get a break in her schedule she'll go and support him at a gig," the source added.
Pals noticed that they seemed close in Beverly Hills last month, when they met to work on Rihanna's new album.