Can Federer hold off Djoker for world No. 1?

Fall is the time when tennis takes a nosedive off the radar, even though the calendar is no less loaded than at any other time of year -- and not just with ATP 250 and 500 events. Two of the nine Masters events (Shanghai and Paris) take place in the fall, and then there's the year-end ATP Tour championships and Davis Cup final.

But then, the Grand Slams are behind us and the players run out of steam as fatigue and injuries accumulated throughout the year take their toll. All of that is good news for those who are opportunistic, strong, healthy and willful enough to suck it up and make an effort to beef up their resumes and bankbooks.

Last year, Andy Murray -- already an established top player if not yet a Grand Slam champ -- went on an autumn tear, winning three events (including Shanghai) and set himself up for what would be the best year -- by far -- of his career. But lesser lights can also flourish in the fall, as Janko Tipsarevic demonstrated in 2011.

Tipsarevic was 27 and had yet to win his first ATP singles title at this time last year. Then he won at Kuala Lumpur and Moscow, and he added a final in St. Petersburg -- wins that set him up for entry into the vaunted top 10 by the following spring. He's done a good job remaining there, although the hard part comes now, when Tipsarevic has to defend all the ground he gained.

So let's look at three intriguing storylines that could mature and heat up by the time the tour wends its way to London and the ATP World Tour finals.

1. Can Novak Djokovic surge and overtake Roger Federer to finish the year ranked No. 1? Presently, Djokovic trails Federer by fewer than 1,400 ranking points. If he wins one fall Masters and loses in the final of another, he'll earn enough points (1,600) to surpass Federer's current total.

Granted, Federer has added Shanghai to his schedule this year; any points he earns there will be added to his total. But he also has to duplicate his wins in Basel, Paris and at the World Tour Finals in order to not lose any points; that's a whopping total of 3,000 points he has to defend, while Djokovic is defending just 560.

Right now, Djokovic is entered in just the two Masters events and the World Tour Finals. He doesn't have much room for error, but anything less than another outstanding fall will leave Federer vulnerable if Djokovic plays up to form.

2. Is there another Tipsarevic out there, ready to step out and make a fall breakthrough? The list of suspects is a long one, given that the baseline objective is cracking, or re-entering, the promised land of the top 10.

Richard Gasquet has been as high as No. 7, and at No. 12 presently, he'd like nothing better than getting back into that elite company. Tommy Haas, once as high as No. 2 but never a Grand Slam champ, has had a remarkable comeback. (He's now No. 21 and has been playing terrific tennis.)

Sam Querrey, No. 26, also is still on the comeback trail and recovering the form that once had him at a career-high No. 17. The current No. 17, Kei Nishikori of Japan, is an icon in Asia, and that ought to inspire him. But the player I see as the best candidate for an autumn breakout is Milos Raonic, who's just 21 and at a career-high No. 15.

3. Can Murray sustain his momentum and close the significant gap that still separates him from Djokovic and Federer? It seems a tall order, but then Murray had to pull out of last year's tour finals, and he's defending just a little over 2,000 points. He ought to be able to accumulate that many if he puts his mind to it, and anything more could put him in position to make the most of any lapse by Djokovic (whom he trails by about 2,000 points) or Federer (who has a lead of more than 3,000 points on Murray).

The problem for Murray is likely to be motivation. He's had an eventful year, becoming an Olympic gold medalist in singles and -- finally -- a Grand Slam champion.

The leaves in many parts of the world are already turning colors, but it's always summer somewhere, and for a tennis player, that's both the dilemma and the opportunity.