Thursday Blues is a regular feature here. This is a spectacular version by an amateur rugby player (you can see some scratches on his face, which is par for the course in the scrum).
It seems that when Bill Withers wrote the song it was unfinished the first time that he sang it with a studio band. He'd intended another verse but it wasn't done, which is why he sang the part with "I know" repeated 26 times. The band loved it and told him to keep it in. Since he was still working a shift at a factory then, he took their advice and something special was born. Still he initially refused to give up his day job (even when this song hit #2 on the charts) because he was worried that the music industry was fickle and wouldn't be there for him long term. One Grammy Award later he realized that maybe this was a better career path.