There's one man standing in the way of the guy who wants to be the last man standing when the ATP World Tour Finals end Sunday in London.
Oh sure, resurgent Rafael Nadal, clever Andy Murray, powerful Stan Wawrinka -- any one of them could theoretically trip Novak Djokovic at the finish line of his most spectacular year. But the adversary who poses the most serious threat, the lurking rival who casts the darkest shadow over Djokovic's sunniest fantasy, is Roger Federer.
And now, thanks to Federer's win over Kei Nishikori in the final round-robin encounter, Federer won't even have to go through Rafael Nadal in the semifinals in order to get (potentially) a final, year-ending crack at Djokovic in Sunday's final.
That Federer has emerged as Djokovic's scourge is a funny thing. After all, Federer is 34 years old. He's already had an historic rivalry with Nadal. And as matchups go, Federer's airy, pretty game would seem vulnerable to Djokovic's heavy, stinging bombardments, never mind that impregnable Djokovic defense.
Yet here we are, with Federer not only capable of matching hits and wits with the far-and-away best player in the world, but threatening to spoil Djokovic's near-perfect run at major events in 2015.
Federer rocked Djokovic in a round-robin encounter Tuesday, 7-5, 6-2. That brought Federer's record against Djokovic on the year to 3-4. But it leaves him in front, 3-2, in best-of-three-set matches.
Federer may be too proud to say or perhaps even to acknowledge it, but 34-year-old guys, even 34-year-old guys with 17 Grand Slam titles to their names, tend to do better in three-setters than five-setters.
Sunday's final in London is best-of-three. Do not for one moment think that Djokovic isn't aware of -- or thinking about -- all of this.
So how does Federer come to be the last man standing in the way of the last man standing? It has a lot to do with a word not often associated with Federer. That word is "tenacity."
It's an odd word to apply to Federer, who is usually described as "balletic" or "silken." When he's in top form, people talk in hushed tones of "full-flight Federer." Like he's some kind of raptor.
But "tenacity" is a grinder's word. It's a word for one of those "good wheels, no serve" players. You want a tenacious critter, you don't think raptor, you think badger. At the ATP level, tenacious is a word put up in lights most recently by Nadal. But Nishikori has a claim on it, too, because he owns the Open era's best winning percentage in matches that go the distance, three or five sets.
It didn't help Nishikori on Thursday. His match with Federer went all the way, but the tenacity of Federer proved superior over the course of three sets in 2 hours, 10 minutes.
Federer led the match by a set and 4-1, when Nishikori began firing those laser-like winners and finding those crazy angles that make him so dangerous. In no time, it seemed, Nishikori broke back and got so puffed up that he rolled through the set, winning 18 of the final 22 points.
But Federer remained cool, even though Nishikori was fighting for his competitive life while Federer, barring miraculous developments, was already all but guaranteed a place in the semifinals. Federer built another 4-1 lead in the final set and Nishikori destroyed that one, too.
Again, Federer weathered a furious comeback to close out the match 7-5, 4-6, 6-4. It was, well, tenacious.
Federer's brand of tenacity requires a supporting framework. And in Federer's case, that begins with his composure. He never lost his head despite blowing leads of 4-1 in those two final sets, just like he wasn't worried about having Djokovic in his round-robin group, or about meeting him in just the second round on Tuesday.
"I was focused more on beating [Tomas] Berdych and [Kei] Nishikori and let's see what happens against Novak," Federer said in his press conference on Tuesday. "That shows me I didn't expect this victory."
Federer's willingness to adapt and change also contributes to his persistence. He's using every tool and every trick in the book to stay competitive. Switching to a racket with a larger head led him through a vale of tears for months back in the second half of 2013. But he resolved to make the transition.
"[The change] gave me a totally different approach on how I can return, how I can serve, what I can do," Federer said.
Federer also has been able to hang tough partly because he's learned to tune out that sometimes overrated voice of experience.
"Experience can also hinder you sometimes because of playing too much percentage tennis," he said. "I feel like I'm young in the mind and I don't shy away from trying new things. That is what keeps it still interesting for me to be on tour."
The things that make life interesting for Federer are a source of anxiety to his rivals, as Djokovic can attest. It would be only fitting if the last official ATP match of 2015 were between top-ranked Djokovic and his tormentor, the tenacious Mr. Federer.