Rihanna, who has topped the charts with a pop-driven R&B sound, has unveiled a new acoustic song in a collaboration with Beatles legend Paul McCartney.

Rihanna unexpectedly late Saturday released the new single, "FourFiveSeconds," which features the Barbados-born singer's signature robust voice but has a minimal backdrop of McCartney's guitar strumming without percussion or electronic elements. The song also features verses sung by rapper Kanye West.

The track is the latest collaboration between West and McCartney, who unexpectedly released a song on New Year's Day after the two stars met each other in Los Angeles.

"FourFiveSeconds" is a song about personal travails and confusion, with Rihanna singing, "We got three more days 'til Friday / I'm just trying to make it back home by Monday morning."

The song could offer hints at the directions of both Rihanna and West, who are separately expected to unveil new albums this year.

Rihanna, who has released seven studio albums, has become an international superstar with electronic-dominated songs such as the ballad "Diamonds" and the danceable "Only Girl (In The World)."

Her music has at times been overshadowed by her turbulent relationship with fellow singer Chris Brown, who pleaded guilty to assaulting her in 2009.