|Russell Crowe at the Oscars|
One said: "They were all over one another and doing little to hide their flirting. They were being really playful and jokey with one another. You could see they really hit it off."
They then went back to Russell's suite with a group of friends where they stayed up until the early hours singing songs around his piano.
The onlooker added to the Daily Mirror newspaper: "They then went back to Russell's room with some friends where they were partying until broad daylight.
"They were singing really loudly together around the piano, belting out some show tunes, and they looked pretty cosy when everyone left them to it."
The morning after Sunday night's Oscars, Russell - who split from wife Danielle Spencer last year - said on Twitter he was feeling the effects of a particularly wild night.
Ahead of his appearance on Jay Leno's show, the father-of-two wrote: "Due to a hangover of massive proportions...anything I say on Leno to night needs to be taken with a pinch of salt...and a slice of lime."
Sources close to Natalie and Russell - who were first linked to each other last month - insisted the pair were "just friends".