They sang show tunes until daybreak, according to reports.
"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," a source said.
They then went back to Crowe'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 source said.
The pair were first seen together at a party last month at Hollywood's Soho House hotel.
They were spotted flirting at the Australian Academy of Cinema and Television Arts Awards, also attended by Robert De Niro and Quentin Tarantino.
Their hook-up comes as the actor prepares for a 15-million-pound divorce from his wife of nine years, Danielle Spencer.
Sources close to the pair, however, insist that the pair are just friends.