A 24-year-old poker professional from Laurel won the World Series of Poker main event, outlasting his final opponents in a marathon card session of nearly 12 hours for the $8.53 million title on Wednesday. Greg Merson emerged with the title before dawn in Las Vegas after a session that proved a showcase for his skills amid the unpredictability of tournament no-limit Texas Hold ‘em. On the last hand, Mr. Merson put Las Vegas card pro Jesse Sylvia all-in with a king high. Mr. Sylvia thought hard, then called with a suited queen-jack.