The 'Closer' hitmaker had a ''surreal'' time in the studio with the 'My Heart Will Go On' star and admitted though he was flattered she wanted his vocals on the track, it made him doubt his abilities as a singer.
He told Digital Spy: ''The first time I worked for her I wrote a song for her, which in itself was surreal.
''This time around I actually did a duet with her - it's her and me on one song, which was a challenge to say the least! I was trying to figure what I was even doing there - why would they need my voice when they have hers?''
''She wanted to do the song as a duet and I was flattered, but it definitely made me question my vocal ability.''
As well as writing and recording with Celine, Ne-Yo has previously penned tracks for a number of other high-profile stars and recently admitted he was delighted some of his songs appear to have made the cut for Beyonce's forthcoming new album.
Speaking following the alleged leak of the record's tracklisting, he said: ''I saw that Beyoncé's tracklisting just leaked and I've got a couple of joints on there, which is good news. I'm very happy about that.''
Asked if he believed the leaked tracklist was real, he said: ''I don't know, I mean you never really can tell until the album comes out.''