By Carl Bialik

Novak Djokovic started 2011 by winning 41 straight matches, mostly by dominating his opponents, including his top rivals. Since Roger Federer ended that streak in the French Open semifinals, Djokovic hasn't been as far ahead of the pack: He's lost five of his last 40 matches and endured some very close scrapes among the 35 wins.

Andy Murray and Rafael Nadal, Djokovic's chief rivals besides Federer, sounded emboldened after pushing Djokovic to five sets in their Australian Open semifinals and finals against him, and it's easy to see why: Each nosed ahead of the No. 1 player in the world late in the fifth set of their matches and could have prevented him from defending his title with just a few more points. On the other hand, Djokovic has shown a newfound ability to win matches with his fighting ability, in a way none of the other members of the Big Four have ever done so frequently on such a big stage.

Consider the last six majors, of which Djokovic has won four — beating Nadal three times and Murray once — lost to Nadal in the U.S. Open final in 2010, and lost to Federer in that French Open semi last year. He has a record of 38-2 in those events. But four of those 38 wins were great escapes: In the 2010 and 2011 U.S. Open semis, in which Federer held two match points in each match but lost, and in the just-completed Australian Open semi and final. In the semi, Andy Murray led on Djokovic's serve, 15-40, at five games all in the fifth set. And in the final, Nadal was up a break at 4-2 and up 30-15 in the game when he missed an easy backhand pass. In each of those four matches, Djokovic returned, Lazarus-like, from the brink, and won, each time by a 7-5 score.

How remarkable is it to come back late in the fifth set of a major semifinal or final against a top opponent? So rare that his three chief rivals have done it just twice, combined, in their illustrious careers. Murray has never won a fifth-set Grand Slam match past the round of 16. Nadal faced a break point while trailing Federer at 3-4 of the fifth set of their classic encounter in the 2008 Wimbledon final. And in the next year's Wimbledon final, Federer saved two break points well into the overtime stage of the fifth set of his epic match against Andy Roddick, then went on to win the set, 16-14. Djokovic himself had never even played a fifth set in a semifinal or final before starting his string in 2010.

To get a sense of how unlikely Djokovic's comebacks have been, I estimated the probability he would win each match at the lowest point in each of the fifth sets. I did so by following the assumptions of Jeff Sackmann, the tennis-stats guru who built a win-probability model on the reasonable basis that the server on hard courts has a 65% chance of winning the point, and 60% on clay. (All the Big Four are better-than-average servers and better-than-average returners, so the averages should roughly apply to them.) By those assumptions, Djokovic had a 9.7% chance of breaking Nadal when he was down 30-15 in that fateful seventh game of the fifth set Sunday, and a 42.7% chance of winning the set and match if he did win the game. He also had a 5.9% chance of winning the match even if he lost the game. Combine those probabilities and he had a 9.5% chance of winning the match at that point. Using the same logic, he was a little more likely to win at his lowest points in the 2010 U.S. Open semi and this year's Australian Open semi, and a lot less likely to come back at last year's U.S. Open semi.

Even accounting for the fact that Djokovic didn't quite come back from the dead in his two losses at majors in that span, winning four of those six tight matches is truly incredible. From the moment in each match when his chances were lowest, Djokovic had about a one-in-4,500 chance of winning at least four of those six matches. That's overstating the case, because it's based on cherry-picking his low point in each match. But it does give a sense of how much Djokovic has had to overcome to survive those four tough matches.

In the end, though, in each of those four matches the better man won. The formula for his three big rivals in each match was to try to win the close sets during dips in Djokovic's play, because otherwise Djokovic was much better. In each match Djokovic won at least set by a score of 6-1 or 6-2 while never losing a set by a score worse than 6-3, and even that, just once. In each match he was at least 16% more likely than his opponent to win points on return, a measure of how much better he was overall. Federer, in their U.S. Open semifinal last year, had the best chance to win but was the most dominated, a measure of how easily Djokovic had won the third and fourth sets before surviving the tough fifth set.

Against Nadal, Djokovic has had to play just one fifth set, in large part because he's been stronger mentally — particularly late in sets. In their last 17 sets of tennis, going back to their two clay-court meetings last spring, Djokovic has broken Nadal in Nadal's last service game of the set 14 times. In all of the rest of Nadal's service games in those sets, he's held 71% of the time, or just about what you'd expect in a matchup between Nadal's strong service game and Djokovic's excellent return game. At crunch time, Nadal has blinked, again and again.

That Djokovic is vulnerable enough to be forced to the brink has made for some very entertaining tennis. It also has made for some very slow tennis, especially when Djokovic meets No. 2 Nadal. Nadal's and Djokovic's final on Sunday was a classic, but it also was about an hour longer than it needed to be, as each player is among the slowest in men's tennis when it comes to pace of play between points. There were 369 points played in 353 minutes of tennis, or 1.04 points per minute, in line with their slow pace in their six meetings in 2011.

You'll hear, in their defense, how grueling the match was. That's certainly a factor — the sight of the two players struggling to remain standing during interminable postmatch speeches before being brought chairs was a memorable one, and in one fifth-set rally they ran a combined 540 feet, or two city blocks, according to ESPN. So it's forgivable that their last 16 points, over the final two games, took 16 minutes.

But even in their very first service games, each player exceeded the allotted 20 seconds between points – on every point. Neither took fewer than 24 seconds even once, and Djokovic averaged 28 seconds while Nadal averaged 26. This was before they'd even played two games. Yet the average rally lasted just 5.4 shots according to Eurosport, far less than the 6.9 shots they averaged during their U.S. Open final last fall. On the same court, in similar conditions and in matches with as many scintillating, long rallies, Federer and Nadal managed to fit in 1.24 points per minute in their semifinal on Thursday. Then Djokovic and Murray squeezed in 1.19 points per minute in their semi on Friday. Had Nadal or Djokovic channeled their inner Federer between points, if not during points, they could have finished the match without setting a new record for longest open-era Grand Slam final.

It truly is a golden age for men's tennis, with four players at the peak, and one at the very top, defeating the other three in matches he had no business winning. The sport just might want to take some steps — Sports Illustrated's Jon Wertheim suggests a shot clock — to ensure it doesn't lose fans, and viewers, during the long periods of down time between these great players' epic points.