Neither Andy Roddick nor John Isner could slay a giant Friday at the U.S. Open .Thus, today’s semifinals feature the undisputed four giants of the men’s game:This year’s French Open winner Rafael Nadal (who dismantled a hobbled Roddick in straight sets Friday) vs. three-time Slam finalist Andy Murray (who took out Isner in four) — No. 2 in the world vs. No. 4.Plus, five-time champion Roger Federer vs. reigning Australian Open and Wimbledon winner Novak Djokovic (No. 3 vs. No. 1) — a rivalry simmering since the French Open, when Federer snapped Djokovic’s 43-match winning streak.