One told the New York Post newspaper: "They seemed to be in an intense flirtatious conversation. They looked really into each other.
"Katie was still at the hotel Sunday afternoon. She was seen with Crowe at the same fireplace at about 2pm."
Sources say the pair have been seeing each other for a few weeks but it's not serious at the moment.
Russell, 48, and 43-year-old Danielle - who have two sons, Charles, eight, and six-year-old Tennyson, together - split in October after nine years of marriage.
Russell regularly praised Danielle, and she was credited with have a calming influence over him.
Shortly after their wedding, the Oscar-winning actor said: "I've been thinking about it for a long time so I don't have any second thoughts. The thought [of marrying her] crossed my mind the first time I met her."
Katie split from Billy in 2009.