MELBOURNE, Australia — For a full two sets and nine games, Roger Federer was the picture of poise, picking up half-volleys with vintage panache and placing winners past Andy Murray. Then, in the bounce of a ball — or two bounces of the ball, depending on opinion — the 17-time major winner was transformed back into the vulnerable, 2013-edition Federer. After breaking Murray to get a chance to serve for the match on Wednesday and a spot in the Australian Open semifinals for an 11th consecutive year, Federer dropped serve.