Rihanna's duet with Chris Martin for the new Coldplay album took months to come to fruition, because the rocker had to pluck up enough courage to ask the sexy singer to lend her vocals.

The Umbrella hitmaker sings on the band's new album Mylo Xyloto on a duet with Martin, entitled Princess of China.

Martin had Rihanna in mind as the ideal guest vocalist, but was too nervous to ask the Bajan beauty when they first met.

He tells Britain's The Sun newspaper, "The album is designed as a whole piece. It has boy and girl lead characters and top of our list for the girl part was Rihanna... but it took a while to pluck up the courage to ask her.

"We did a show in Las Vegas and I met her and said, 'Do you think there's any chance?' I was very Hugh Grant-like and spluttering about it.

"She has such an amazing voice and it's so different to mine. When the vocal happened I thought 'Wow, you sound different' but I think that's also because she was singing a different song to what we are used to hearing from her and it is written by a bloke from Devon (England)."