The two singers enjoyed spending time with one another at the Coachella music festival in Los Angeles last weekend and decided to see each other again in New York a few days later.
A source close to Usher said the pair have been friendly for a long time and have shared some "passionate kisses."
They told The Sun newspaper: "They had a really good time together last weekend and wanted to see each other again somewhere private straight after. They've known each other for a long time but it's always been bad timing.
"They've had a few long, passionate kisses. There's definitely a spark between them.
"It would have been too obvious to go somewhere in LA and they were both going back to New York for work commitments."
Earlier this year the "OMG" hitmaker declared his musical interest in the "S&M" star, revealing he wanted to record a duet with her.
He said: "Of course I'd do a track with her—hold on! We've been friends and our teams work together and we've talked about working together in the past, but it could potentially happen.
"I would love to work with her—I think she's an incredible artist."
