The Charlie's Angels beauty reached the milestone in August (12) and on a night out to mark the occasion, she shared dinner with the Top Gun actor and fellow Hollywood stars Javier Bardem and Penelope Cruz.
As the clock struck midnight, the four screen stars posed together for photographs as a keepsake - but their fun moment unwittingly sparked frenzied speculation of a new romance.
A representative for Cruise spoke out in September (12) in a bid to silence the rumour, and now Diaz has joined her pal in denying they are in a relationship.
She tells U.K. talk show host Graham Norton, "I happened to bump into Penelope Cruz, Javier, Tom and his son in a hotel and joined them for dinner. At midnight I said, 'I'm 40' so we took pictures and then said 'Goodnight, see you later, blah, blah, blah' and I haven't seen him since. But the next thing I know, apparently I am dating him and it's full on and he is coming after me."
Diaz has been single since splitting from baseball star Alex Rodriguez last year (11), and Cruise's divorce from Katie Holmes was finalised in August (12).