The Hitch actress, who is already in a longterm relationship, sparked rumours of a romance with Phoenix after the pair showed up at New York City restaurant La Esquina on Monday night (27FEB06).
A source tells gossip site PageSix.com, "They were absolutely adorable together" and reports they "held hands and danced together" at Club Butter after dinner.
The source adds, "It was weird - everyone knows she has a longtime boyfriend."
However, Mendes' spokeswoman NICOLE CARUSO insists the actress was simply socialising with a work colleague, saying, "They are just friends.
"They are in town doing a movie together called WE OWN THE NIGHT with Mark Wahlberg.
"Eva is still with her boyfriend."