Tomas is a stranger who very briefly appears at the beginning of The One With The Ultimate Fighting Champion with a guy called Tim. They have a bizarre conversation within the coffee shop which is eavesdropped upon by the six friends.
The friends first encounter Tomas and Tim when they interrupt Monica as she is about to tell them a story regarding her relationship with Pete Becker to ask Rachel, Phoebe, Chandler and Joey to make some room on the sofa because all the other seats in the shop had been taken. After they have made some room for Tomas and Tim, Monica attempts to begin her story again, but Tomas interrupts her a second time when he suddenly bursts into tears. When Tim asks what the matter with him is, Tomas claims to suspect his wife is cheating on him, believing the man she is sleeping with to be her gynecologist because "he's got access." They bizarrely compare the feeling Tomas now has whenever he sleeps with his wife to the feeling you get "when you're bowling and you know you're in someone else's shoes." The six bewildered friends try to ignore them and focus on Monica's story, but this becomes impossible even for Monica herself when Tomas, wondering how he could have became so unattractive to his wife, suggests that "maybe it's (his) wound," which "hasn't healed yet" but is "oozing." Tim then remorsefully confesses that HE is the one sleeping with Tomas' wife, causing an understandably confused and curious (though at the same time rather crude and thoughtless) Joey to ask if Tim is the gynecologist Tomas was earlier referring to, only to be told to mind his own business by Tim. Tomas then bursts into tears again as he absorbs the fact that Tim has been sleeping with his wife and begins shouting at and insulting Tim, and ends their friendship before leaving the shop in a loud fury. Tim follows him out, giving the six shocked friends one last apologetic, embarrassed look before he leaves. After a stunned silence, Phoebe asks Monica what she had been about to say before Tomas and Tim interrupted them. Understandably, Monica can no longer remember.
Tomas was portrayed by famous actor Robin Williams, while Tim was portrayed by famous actor Billy Crystal. Both actors, who are also comedians, ad libbed the bizarre conversation their characters have within the coffee shop, which wasn't in the original script. Apparently, they happened to be in the same building where the studio was located and were asked if they would like to make cameo guest appearances, which they agreed to do. Neither of them were credited, however.