Only four players remaining in the men's draw at the 2015 U.S. Open: Novak Djokovic, Roger Federer, Stan Wawrinka and Marin Cilic. Federer is playing some of his best tennis in recent memory. He is efficient on court, taking balls early and keeping rallies shorter. All of the other men have played longer matches and have played more games, hence why they’ve run further so far in the tournament. It will be intriguing to see if Federer can keep up his efficient play as the opponents get tougher in the men’s semifinals. Who will advance to the finals: the players with fresher legs or the ones with a few miles on them?

Check SI.com's 2015 U.S. Open data hub page throughout the whole tournament for the latest data-driven infographics and charts from IBM, the official technology partner for the U.S. Open. IBM captures and analyzes the data that powers all of the Grand Slams, as well as the digital platforms that extend the experience to fans around the world.