What happens when an unstoppable force meets an immovable object'The closest we can get to answering that question (at least in tennis) is the Djokovic vs. Federer rivalry.Every time these two colossal figures collide, the stakes couldn't possibly be any higher. They've already played one another a whopping 43 timesthe second most frequent matchup in tennis historywith 17 of those occasions coming in finals and another 21 happening in semifinals.Because of how late they usually meet in tournaments, their matches drip with tension and take on added importance.And the latest installment in their seriesa 7-5, 6-2 win for Federer at the World Tour Finalsunderscores just how competitive a rivalry it's been. Unlike the Federer vs. Rafael Nadal series (which Nadal comfortably controls) or even the increasingly one-sided Djokovic vs. Nadal battle, this one is more evenly balanced.Though their first four matches went Federer's way, neither man has produced a streak longer than three wins against the other since 2007.As Djokovic matured into the force he is today and Federer dealt with the natural aging process, both players have seen ebbs and flows in their careers. Yet no matter where they are with their games at any given moment, they have an uncanny knack for bringing out the best in one another.They've given tennis some of its most legendary matches, including their epic 2014 Wimbledon final, five-set thrillers at the U.S. Open in 2010 and 2011 and so many other indelible moments on stages big and small.In a piece examining their rivalry, FiveThirtyEight.com's Carl Bialik argued for it as the greatest in men's tennis history, writing:Federer-Djokovic provides plenty of intangible pleasure for neutral fans.Each can defend but prefers to play the aggressor, making for quick reversals and for many unpredictable points. Federer has one of the sports most effective serves ever. It has a mighty foil in Djokovics possibly best-ever return. Federers rekindling of his love for the serve-and-volleyraises the tension.Bialik hit on some key tactical factors which help make Djokovic vs. Federer so aesthetically pleasing.For starters, both men play radically different games.Federer is the constant aggressor and a practitioner of the one-handed backhand. With his get-to-the-net mentality, he's like a cheetah eager to pounce at the first opportunity. Djokovic, with his two-handed backhand, is more like a hawk. He patiently surveys his surroundings, using his stellar defense to tire opponents from the baseline before zooming in on his prey.Despite their six-year age gap, Federer, 34, and Djokovic, 28, are each superbly conditioned and possess a unique set of talents. The dichotomies in their games are notable, but as the saying goes, opposites attract. And their differences only make the rivalry all the more interesting.Underneath it all, there's a not-so hidden layer of tension between the two. Though both men clearly respect one another, their personal relationship seems chilly at best. You can chalk most of that edginess up to their competitive natures, but there's no denying they get under each other's skin.After his round-robin loss to Federer in London, Djokovic touched a nerve in his opponent by saying he "handed" him the win. The Swiss star retorted by telling reporters (h/t Chris Chase ofFor the Win):"He handed [me] the win' ...Its not like he played terrible. I know he can play better. Why did he play that way' Id like to give myself credit for that, quite honestly, yeah."Not exactly earth-shattering material, but that exchange is enough to illustrate how little love is lost between them. Which might be good for tennis. In a sport that (fairly or not) has a reputation for being too formal, it's actually refreshing to see two players who maybe aren't so friendly when they face off. It adds an element of drama to the proceedings.When Federer beat Djokovic this week, he earned some emotional payback for his loss in the U.S. Open final and took the ever-so-slight lead in their head-to-head record. That win also stopped a number of notable streaks by the Serbian.#Djokovic streaks come to an end: 38 indoor match wins 23 overall match wins 15 #FinalShowdown wins #ATPWorldTourFinals Greg Sharko (@SharkoTennis) November 17, 2015Also factoring in to what makes their rivalry so great' The historical implications. Thanks to Djokovic's mid-career surge, he now owns 10 major titlesseven behind Federer's record. When the Swiss took that honor from Pete Sampras in 2009 and kept stretching his mark, it appeared safe for the foreseeable future. Then along came hard-charging Nadal, who won his 14th Grand Slam title at the 2014 French Open. Cue Federer beginning to sweat.It seemed inevitable that Nadal would challenge (and probably eclipse) Federer at that point, but his momentum vanished because of injuries and a subpar 2015 season. He's finally getting back on the right track, yet Djokovic may have surpassed him as the man most likely to break Federer's record.Sure, seven majors is still a sizable deficit, one that would require a few more dominant years from Djokovic to overcome. Based on his current form, it's not that outlandish to suggest he can do it. After all, he very nearly won the calendar Grand Slam this season, and the world No. 1 is in the absolute prime of his career. Barring major injuries, Djokovic's reign won't be ending any time soon.Whether or not he can get there is a question that will heat up in the coming years. Regardless of that record, both men have established themselves as two of the absolute best to have ever played the sport. Maybe even the top two before it's all said and done.If we are truly witnessing the golden age of men's tennis, then the Federer and Djokovic rivalry should serve as the foundation for what makes it so great. Athletic and unpredictable matches, bitterness between the combatants and legacies on the linethere's not much more these men could do to up the ante higher.Should they square off for the title this Sunday at the World Tour Finals, another important chapter in their story will be written.All statistics are courtesy ofATPWorldTour.comunless otherwise noted.JoeKennardis a featured columnist for Bleacher Report.Follow @JoeKennard
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