Through four matches, No. 2 seed Roger Federer has been performing as expected, but the great revelation has been No. 20 seed Gael Monfils. Famously entertaining but famously inconsistent, he's playing some of the best tennis of his 10-year career at the 2014 U.S. Open. Federer leads their head-to-head 7-2, with both Monfils victories coming indoors on hard court, but what does the data say about their match-up? Here's an in-depth look at both players heading into their quarterfinal match.