The logical extreme for a diplomatic invitation to gamble, assuming you're not a hustler but an honest person looking for fair action ----
"Forgive the interruption. My name's John. I wanted to ask you a question and if you say no, I'll understand in full. I was wondering if you might enjoy playing a match with me for a few dollars. I've seen you play and I think the match would be close. You'd be doing me a favor, for in my experience, the pressure of a small wager helps me to perform at a higher level. As I said, though, if you'd rather not, that's OK." The invitaiton may be accepted, but if the player does not wish to gamble, say "have a nice evening and good luck with your practice and, once again, I'm sorry to have interrupted you."
No matter how this turns out, you have treated this person with respect.
If someone talked to me like that in a pool hall I'd think he was a psych case.