The 47-year-old British singer had been suffering with a tooth infection since Monday and took to Twitter on Thursday night to tell his fans that he would be in no fit state to perform.
'This tooth ain't getting better in time for tonight, not enough to sing anyway,' he wrote.
'I'm really sorry folks. You watch, it'll be totally fine by tomorrow. Bloody typical.'
But he quickly added, 'Of course I'd recommend going anyway, Alicia is great live, and @KeepaChildAlive is great charity to donate to.'
But according to the Daily Star, the news has not gone down well with Keys.
'Alicia has a young baby to look after, so this last-minute change to her big night is the last thing she needs,' said an insider.