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The true rivalry: Djokovic-Nadal

Novak Djokovic and Rafael Nadal have renewed their rivalry © Getty Images

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Don't look now, but the big four whom everyone has been jabbering about for a couple of years now has suddenly shrunk to a big two, which is rivalry as it was meant to be in the gospel according to Pete and Andre or Chris and Martina.

Just a few months ago, as current ATP world No. 3 Andy Murray won Wimbledon to make history, claim his second major title and add credence to the idea of a big four, we were talking about this new golden age of tennis. Now, we're wringing our hands in the wake of Roger Federer's disastrous attempt to salvage his summer (and subsequent plummet to No. 5) and out of fear that Murray's theoretically minor back surgery might prove less "minor" than anyone thought.

And that leaves us with just two men standing, top-ranked Novak Djokovic and No. 2 Rafael Nadal. And one of them (can you guess which one?) looks very unsteady on his feet.

How did this happen so quickly?

Well, age seems to have caught up with the 32-year-old Federer faster than anyone expected. It wasn't so much that the defending champ was upset in the second round at Wimbledon by Sergiy Stakhovsky. It was what came afterward: Federer's truly bad losses in European post-Wimbledon clay-court tournaments while he floundered with a new, larger-headed racket and then an ugly, mortifying loss to another over-30 player (Tommy Robredo) in the fourth round at the US Open. Whoever thought the name "Federer" and "ugly" could wind up in the same sentence?

But Pete Sampras and others can tell you that aging in tennis is never a pretty thing. The only way to get out with your reputation fully intact is to quit while you're on top, and that's neither easy (who knows when you're truly done?) nor fair to yourself. As Billie Jean King has said, you owe it to yourself as a human being to experience the truths and lessons that come with decline. It seems that Federer is living out that process now.

Murray's situation is a little more complicated and perhaps more interesting. He was accorded big-four status mainly on the strength of his record in Masters events (nine titles) and the encouragement he received from his big-four peers as he struggled to win that first major final - a feat he accomplished in his fifth try, at the US Open in 2012.

The reality is that Murray, while he's been ranked as high as No. 2, has always been the laggard in the big four. That doesn't mean he's some sort of fake; he's clearly a better, more able competitor than, say, Tomas Berdych or Jo-Wilfried Tsonga. But he's never really been able to establish parity with those other three guys who have taken turns beating up on each other.

The proof is in pudding of the rankings. Murray trails No. 2 Nadal by nearly 4,000 ranking points. That's four Masters 1000 titles or two Grand Slam titles worth of points. That wouldn't be so bad if there weren't a couple of players bearing down hard on Murrray. ATP No. 4 David Ferrer trails Murray by just 400 points, and Berdych and Juan Martin del Potro, a former US Open champ, are not far behind.

Ferrer is sure to surpass Murray within weeks because Murray is out for the Asian circuit, perhaps the entire year. He's losing over 1,200 ranking points if he pulls the plug on the entire year, meaning that Federer, Berdych and del Potro will have a good chance to pass him in the rankings while he recuperates.

Murray traditionally plays well in Australia, the most likely site of his return. (He's a three-time runner-up at the Australian Open, the first major of the year.) But it's still hard to dodge the feeling that at the start of 2014, there will be no trivalry, nor a quadvalry. It looks like Nadal and Djokovic will turn a four-man photo op into a two-man race.

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This article first appeared on ESPN.com

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