Andy Murray and Novak Djokovic, based on form from September onward, are likely the two hottest players on the men's tour. Djokovic made it two straight wins against Murray by topping the Scot on Day 3 of the ATP World Tour Finals on Wednesday. In three sets, of course.
With their athleticism, retrieving skills and ability to turn defense into offense -- and Murray turning into a Grand Slam winner -- the rivalry is growing in stature.
But is it the best current rivalry among the big four? Not quite.
Here's our list:
Righty versus lefty. Elegant brilliance versus power, will and the heavy spin. Two of the greatest players of all time.
How can it still not be Roger versus Rafa?
They've played some duds in the past (see the 2008 French Open final and last year at the World Tour Finals), but lopsided encounters have featured in all six of the big four combinations.
They played probably the greatest match ever, at Wimbledon in 2008, which counts for a fair chunk, and their most recent Grand Slam head-to-head, in Melbourne in January, was entirely absorbing.
With Federer still going strong at 31 (and vowing to keep playing for a while) and Nadal's knees on the mend, we can hope for another meeting in a Grand Slam final, perhaps even at the U.S. Open. New York, sadly, has been deprived of a Federer-Nadal blockbuster.
2. Djokovic versus Federer
Federer leads series 16-12
The rivalry with an edge to it: Remember Federer telling Djokovic's parents to be quiet at the Monte Carlo Masters in 2008? Or Federer questioning Djokovic's tactics after the Serb saved a match point at the 2011 U.S. Open with a blistering forehand return winner?
"I believe in hard work's gonna pay off kinda thing, because early on maybe I didn't always work at my hardest," Federer said. "So for me, this is very hard to understand how [you] can play a shot like that on match point."
Djokovic saved two match points a year earlier in New York for even more drama.
3. Nadal versus Djokovic
Nadal leads series 19-14
When two of the finest movers in the game battle -- Murray is in the mix -- the result can be some very, very long matches. Since Nadal and Djokovic don't possess the most dominant serves and return well (Djokovic better than Nadal), breaks arise and proceedings are prolonged.
Six hours of tennis isn't to everyone's liking, but this season's Australian Open final -- the third of four consecutive occasions they lined up in a Slam finale -- had to be classified, at the least, as gripping.
Before his knees gave way, Nadal ended a seven-match losing streak to Djokovic and has now won three straight in the rivalry.
4. Federer versus Murray
Murray leads series 10-8
Not as spicy as Federer-Djokovic, but this one has a kick to it, too. The British media had a grand old time several years ago when Federer said Murray's play was too defensive. (Wasn't he right?) And Federer resorted to mind games ahead of the 2010 Australian Open final -- not something he'd do against Nadal -- when he reminded Murray that the U.K. hadn't produced a men's Grand Slam winner in "150,000 years."
Federer's animosity has mostly disappeared, and his respect for Murray is apparent. Although the Olympic final lacked tension, the Wimbledon final didn't.
With Murray now expected to finish second in Group B at the World Tour Finals and Federer favored to win Group A, a third match on U.K. soil in 2012 could be on the way.
5. Djokovic versus Murray
Djokovic leads series 10-7
Similar to Nadal-Djokovic, except Murray's reputation isn't as hefty as Nadal's. The rallies are extended and striking a clean winner is difficult; Murray adds class with his touch and variety.
It's simple to get carried away, since Murray and Djokovic have played three times since the start of the U.S. Open. But Murray needs to collect more majors to give more weight to the matchup.
6. Nadal versus Murray
Nadal leads series 13-5
Unlike the other five pairings, Nadal and Murray haven't lined up in a Grand Slam final. That's not to say tussles are dull or insignificant. Far from it.
As in Nadal versus Djokovic and Murray versus Djokovic, when watching Nadal and Murray go at it, settle in. You'll be in your seat for a while. Nadal edged a three-hour thriller at the World Tour Finals in 2010, and even a 3-6, 6-2, 6-0 score in Tokyo last year (a Murray win) resulted in a duration of 2 hours, 16 minutes.