Talk about a sucker punch. When overly eager agent Josh Klein, 26, tried to lure actor Mark Wahlberg away from his current reps, the two agreed to an unorthodox negotiation: If Klein could last three minutes in the boxing ring with Wahlberg (right), the Italian Job star would ditch his agents at Endeavor and sign with his buddy Klein at United Talent Agency. One witness says Klein should have thrown in the towel as soon as he laid eyes on the shirtless Wahlberg, who hosted the Aug. 25 match in his own ring (the actor has a full gym in his L.A. home). ''Mark is big,'' says the witness. ''I don't know if I would have gotten in the ring if I were in my car.'' Still, Klein, who weighs about 40 pounds less than Wahlberg, strapped on the head-gear and came out swinging. Not that it mattered; Klein barely got a punch in before Wahlberg knocked him down three times. ''The last one was to the chest,'' the witness says. ''Josh let out a squealing noise.'' Though he lasted only a minute, Klein got a lifetime of respect from UTA. (''We obviously don't condone the pugilism, but at least he didn't chicken out,'' said an agency spokesman.) As for Wahlberg, who couldn't be reached for comment, he hasn't signed with Klein -- yet.