The funnyman was said to be 'distraught' after learning Italian film producer Martino Sclavi had a brain tumour.
According to the News of the World, he pulled out of an appearance at the British Comedy Awards, where he was due to win the accolade for his 'Outstanding Contribution To Comedy'.
Instead, he rushed to the bedside of his friend at a New York hospital, where he had just undergone surgery to remove the tumour. His condition is said to be critical.
Appearing via video message, he told the awards audience: 'I'm really sorry I couldn't be there. One of my friends is really ill and there's no way I could travel.'
A source added to the newspaper: 'Russell would love to have had some wild fun on stage with Jonathan [Ross]. But sometimes there are more important things in life. It was a real shock when Martino fell ill. Russell was very shaken up. But he has remained with Martino in hospital since and will do everything he can to make him as comfortable as he can.'