Speaking to The Daily Telegraph, Mendes said that he thought Craig was wrong for the role before seeing Casino Royale.
"I thought he might be wrong to play the part, I thought he might be the wrong casting," he explained.
"And it took me about 30 seconds of Casino Royale to realise: wrong. He's a brilliant Bond and he completely changed it. He hasn't made it better or worse, he regenerated it."
Mendes also revealed that it was Craig himself who suggested that he direct a 007 instalment, saying: "It wasn't until Daniel asked me, 'Do you fancy doing a Bond movie?' and I had a feeling immediately, and I'm so glad he asked me the question."
lets audiences into Bond's inner life.
> 'Skyfall' premiere: Daniel Craig, James Bond cast in London - pictures
> Daniel Craig: 'Skyfall' star's top five screen roles
Skyfall is released this Friday (October 26) in the UK, and November 9 in the US.
Watch a trailer below: