The heartthrob actor - who once dated Nicole - says the flame-haired beauty is head over heels with the country singer.
He is quoted in Britain's The Sun newspaper as saying: "I've spoken to Nicole a couple of times recently and it was clear she's enjoying great happiness right now.
"Hanging out with Keith is giving her a lot of joy and I'm happy for her."
However, Crowe fears the high-profile couple will eventually implode - the way his romance with actress Meg Ryan did - because of the intense interest in their relationship.
He added: "A relationship under the microscope doesn't get the chance to evolve. It happened to me some years back. My summer was ruined as we were never left alone."
Last month, it was claimed Nicole and Keith were getting serious after inviting their parents to spend Thanksgiving with them.
The actress reportedly flew with her parents to Keith's Tennessee home for the celebration.
The following day, Nicole and her father were seen out jogging, while Keith and the two mothers took a stroll together.
The Canadian Press BANG Media International
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