Roger, Rafa to break new ground in round-robin match

Roger Federer and Rafael Nadal have played each other three times in the Tennis Masters Cup/World Tour Finals, but never in the event's round-robin stage. That will change next week, as both men were drawn into Group B, which also includes Jo-Wilfried Tsonga and Mardy Fish.

Since 2005, the first year both Federer and Nadal qualified for the season-ending tournament (even if Nadal didn't actually play it), it's been impossible for the two rivals to meet at this juncture—they had always been ranked No. 1 and No. 2, in some order, and therefore couldn't be placed in the same group. But Novak Djokovic's top-drawer season allowed the possibility of a round-robin rumble this year. It will be one of the hottest tickets in London, the first time since the third round of the 2004 Miami Masters—their first match—that Federer and Nadal will square off before the semifinals of a tournament.

History and current form favor the presently lower-ranked Federer. The five-time champion is fresh off two indoor titles—Basel and Bercy—and Nadal, who last played in the third round of Shanghai, has rarely thrived inside. He's also never beaten Federer at this event, losing all three meetings, including last year's final:

Nearly half of Federer's eight wins against Nadal have come at the season-ending finale, but he's also downed the King of Clay at Wimbledon (twice), Miami, Hamburg and Madrid. Nadal's 17 triumphs over Federer have been earned at nine different tournaments (if you count Hamburg and Madrid as two), including three of the Grand Slams.

No matter the result of their 26th match, the round-robin setting makes it a first—a rarity for these tennis titans, who've contested each other in almost every imaginable scenario. Except for a few. Here's five Roger-Rafa happenings we're still waiting on:

1. They've never played at the U.S. Open
   And you wonder if they ever will, given the opportunities Federer squandered over the past two years—four, to be exact. That's how many combined match points Federer had in two consecutive semifinals (facing Djokovic each time), a pair each year. Miraculously, Djokovic denied them all, won both matches and earned two showdowns with Nadal. The Serb is tennis' answer to Yogi Berra, who famously coined the phrase, "It ain't over till it's over."

For years, Federer, who reached six straight U.S. Open finals from 2004-09, was waiting for Nadal to show up at the end—the Spaniard didn't hold up his end of the bargain in 2008 and 2009, losing in the semis. But now the script has been flipped. Maybe it's just not meant to be.

Odds of this happening: 10 to 1

2. They've never played in between Wimbledon and the U.S. Open
   When Roger and Rafa met in the semifinals of this year's Miami Masters, it was the first time they'd collided in North America since 2005, also in Miami. It was hardly worth the wait—in one of their most uncompetitive matches, Nadal crushed Federer, 3 and 2:

Still, Key Biscayne was the scene for one of tennis' greatest rivalries for a third time. Toronto, Montreal and Cincinnati, hard-court Masters tournaments that precede the U.S. Open, are still waiting their turn. (So is Indian Wells, held right before Miami.) It's surprising that North American encounters between the two have been so rare, despite the later rounds often being their only possible rendezvous point. Consider: They've played the Monte Carlo final together three times, the Rome final once, and in Hamburg or Madrid five times—all clay-court tournaments, where Federer typically has his "worst" results (he went 2-7 in those matches). Chalk this omission up to post-Wimbledon fatigue and the fact that, if Nadal has a "worst" surface, it's hard courts.

And just so we're clear, "worst" for Federer and Nadal equates to the best for almost every other professional tennis player.

Odds of this happening: 5 to 1

3. They've never played in Davis Cup
   This missing puzzle piece could end up being the toughest to find. Federer's participation in Davis Cup has always been spotty, and when Spain and Switzerland were drawn to meet each other in the opening round last year, both Roger and Rafa took a pass. It was a bummer, but not wholly unexpected.

Another potential roadblock: Even if their respective countries collide, there's a chance, albeit a small one, that their likely fourth-rubber match could be rendered irrelevent, dead, if one nation sweeps the first three. I'm sure they'd still play it out, but it wouldn't be the same, and we've had our fill of Federer-Nadal exhibitions, including the half-grass, half-clay Battle of the Surfaces in 2007:

The best chance of a Roger-Rafa rubber is in the competition's final two rounds, when both men would likely make themselves available. That's the earliest they could meet next year, but that's a long ways away on mutliple levels. In the meantime, lend your support to Davis Cup stalwarts Stanislas Wawrinka and David Ferrer, who will try and steer their ships toward a potentially memorable semi.

Odds of this happening: 20 to 1

4. They've never played on grass beyond Wimbledon
   And maybe that's for the best, to keep their three straight SW19 finals so hallowed. But their last one, the 2008 epic, feels like a long time ago, and I'm sure some of you want to see another R/R chapter unfold on turf, at the All England Club or elsewhere. You may get your wish: Nadal, who traditionally plays Queen's Club, will instead play Halle in 2012. Federer has missed that Wimbledon warm-up just three times in the last 12 years, and has a playing agreement with tournament organizers.

Odds of this happening: 3 to 1

5. They've never played together in doubles
   I might want this to happen more than any of the others. Federer and Nadal exposed tennis to more people, improved the overall quality of play, and are outstanding ambassadors for the sport. They get along very well, and it seems natural that they should, at some point, literally join forces.

Obviously, they'd be a formidable pair—they'd even have the lefty-righty dynamic working. Sort of like Nadal and Djokovic, who teamed at last year's Canadian Open. (It should be said that they lost their only match there.) Both Roger and Rafa ocassionally dabble in doubles, though usually with their countymen. A Spanish-Swiss union seems unlikely in 2012, given the busy schedule with the Olympics, but I suspect at some point they'll play on the same side of the net. Maybe at a tournament they haven't won in singles—I'm racking my brain just to think of one. For now, we'll always have this, from the Rally for Relief in Melbourne earlier this year:

Odds of this happening: 2 to 1

Ed McGrogan is the online editor of TENNIS.com.