Prepare for the unpredictable in London

OK, as we hurtle toward the year-end Barclays ATP World Tour Finals, let's see if we have this straight:

Rafael Nadal wins the season's first Grand Slam, then limps on aching knees back home to Mallorca. Roger Federer takes the next two majors and then, with typically exquisite Swiss timing, becomes a father (twice) -- all in the span of six weeks. Juan Martin del Potro, only 20, wins his first Grand Slam title, the last of the year, in New York. While everyone else is shutting it down, Novak Djokovic puts on a burst at the end, defeating Federer and Nadal. Then, after losing in four ATP World Tour Masters 1000 finals, he beats Gael Monfils in the Paris final on Sunday.

Afterward, Djokovic was asked whether he felt he was the best player on the circuit.

"No," he said, "I don't feel as No. 1 until I am No. 1. I want to perform my best on every match I play. Some critics could say that it's wrong for [Barclays] in London.

"Still, I think it's good, because who knows what's going to happen in one week? You live for today."

Ah, carpe diem. Who will seize the prize in London? It depends on the day. The logical favorite, based on the recent chaos, would have to be Andy Murray, the only top-5 player who has yet to fully assert himself.

Parity, once the peculiar pride of the NFL, seems to have taken hold in men's tennis. Kind of refreshing, isn't it?

"I think that you've probably found the ideal time for everyone in the individual stages of their careers," said the former top-10 player Todd Martin, who, along with Marian Vajda, coaches Djokovic. "Federer has dominated for so long, and so has Rafa, though he has had a tough go physically. Murray and Djokovic have established themselves as mainstays, and del Potro has come along.

"Now you're starting to see a more level field. It's nature. The big guys are becoming the hunted -- and the younger guys are starting to track them down."

Between 2004 and 2007, Federer won 11 of 16 majors. Even though, at age 28, he won two this year and reached all four finals, his forehand -- the window to his uncertainty -- consistently has let him down. Going forward, his fifth-set losses to Nadal in Australia and del Potro at the U.S. Open may be more telling than his 16-14 triumph over Andy Roddick at Wimbledon.

Still, the resilient Federer is on the cusp of doing something rare. He has a comfortable lead for the No. 1 ranking and should become only the second man to finish on top after losing it for one year, following Ivan Lendl in 1989.

Nadal, last season's year-end No. 1, is the only one who could take it away. His best scenario: win the title with an undefeated 5-0 record while Federer goes 2-1 in round-robin play and falls in the semifinals.

Federer and Nadal were easily the top year-end points leaders, followed by Djokovic, Murray, del Potro, Roddick and Nikolay Davydenko. In fact, the only change from last year's final eight qualifiers is that Fernando Verdasco displaced Jo-Wilfried Tsonga. Verdasco qualified for his first year-end event when his main rivals for the last spot, Tsonga and French Open finalist Robin Soderling, lost in the quarterfinals at Paris.

It was appropriate that Verdasco, who lost to Marin Cilic in the third round, did not win his way into the field at the BNP Paribas Masters; it was a microcosm of the unpredictable season.

To review: Roddick pulled out before the tournament with a knee injury, Federer lost to Frenchman Julien Benneteau in the second round, Murray fell to Radek Stepanek in the third round, Del Potro retired (abdominal strain) from his match against Stepanek, and Nadal lost to Djokovic in the semifinals.

Djokovic was the last man standing, but that means he'll have less gas in his tank than any of the other competitors. He has played (94) and won (76) more matches than anyone on the men's side.

"Tomorrow morning," Djokovic said in Paris, "my fitness coach was planning to have a two-hour practice. I'm joking, of course."

Djokovic said he was heading home to Monte Carlo for a few days and hoped to attend Wednesday's soccer friendly between Serbia and South Korea in London.

"Frankly, right now, he still has his wits about him, which is difficult to do at this time of year," said Martin, who will spend Thanksgiving away from his family -- instead, he will be encouraging Djokovic in London. "I've heard questions about his heart, but from what I've seen he has a huge heart and fights like a dog. He's won with grit and determination.

"Yes, he's played a lot more matches than anyone -- I don't necessarily think that's a good idea. But he's extremely fit. That's his greatest weapon right now."

Since he has a horse in the race, Martin declined to specifically handicap the Barclays field.

"It's such a unique opportunity to compete against your most esteemed peers," Martin said. "I think despite what you've seen recently, the guys will be ready to play. That being said, I think you have to be prepared for something different than the predicted results to occur."

Greg Garber is a senior writer for ESPN.com.