Criss met Holly around 2 a.m. at Cathouse at the Luxor hotel after doing two performances of his new show Believe. The duo was seen drinking, dancing and canoodling until 3:30 a.m.
And although Holly has said she's "just friends" with the magic man, a source counters that they are "very, very good friends."
"They are both so freaking busy that seeing each other is tough," the source tells E! News. "But they are working harder to spend time together."
Holly is expected to walk the black carpet Oct. 31 for the opening-night gala of Criss' show.
—Reporting by Whitney English