Roger Federer isn't the best singer, and he admits it

Roger Federer says he didn't really know the lyrics so that's why he may have been a little off while singing in tennis' newest boy band.

"I thought it was terrible acoustics in that room you know," Federer said when asked on court by Jim Courier about singing Chicago's "Hard to Say I'm Sorry" with Tommy Haas and Grigor Dimitrov. Federer beat Kei Nishikori in five sets to advance to the quarterfinals of the Australian Open.

"That’s why I think my voice was going up a little bit," he said of not really knowing the lyrics.

Haas is the best singer among the group, Federer said. The piano player, David Foster, who co-wrote the song, was the big musical talent.

"But I think Grigor – he looks good; his voice is terrible," Federer said.