Just when it didn’t seem to matter, when the players didn’t want to play and the tours were hoping to put another bloated season to rest, tennis fans were given a reason to watch again yesterday.

At the most basic level, they got a well-played, five-set match in the final of one of the game’s major events, the ATP’s year-end Masters Cup. David Nalbandian came from two sets down to upset world No. 1 and two-time defending champion Roger Federer in a final-set tiebreaker, but not before Federer did “what all great champions do,” rallying from 0-4 down in the fifth to come within two points of the title. While Federer was tired and hampered by an ankle injury, Nalbandian showed again that he’s a paragon of perseverance. Who—other than Rafael Nadal—would have the mental strength to lose a second consecutive tiebreaker to the world’s best player, 11-13, and put it behind him quickly enough to win the next set 6-2?

Doggedness aside, Nalbandian may have been rescued in the end by an overrule that went against him. Riding a surge of momentum, Federer served for the match at 6-5 in the fifth. He went up 15-0 and hit what appeared to be an ace. The line judge called it out, but the chair umpire reversed the call and announced the score, 30-0. Nalbandian protested, but not too mightily. Instead, he hit a backhand winner with his next return of serve. From there he broke and won the tiebreaker with relative ease. The umpire did Nalbandian a huge favor: He gave him something to fight about, which took his mind off his missed opportunities.

It was Federer’s fourth loss of 2005, leaving him one win shy of the best single-season men’s record. John McEnroe went 82-3 in 1984; Federer went 81-4 this year. Those four slip-ups came against Marat Safin, Richard Gasquet, Nadal, and now Nalbandian. Does that lineup tell us anything about what it takes to beat Federer? Two possibilities emerge: (1) Other than Safin, these players don’t have huge serves. It isn’t essential because, no matter how much pace you generate, Federer gets more of them back than any other player. (2) What does seem essential is having a very strong ground stroke on your left-hand side, which allows you to counter Federer’s most important shot, his inside-out forehand. That’s the backhand side for Safin, Gasquet, and Nalbandian, and each of them is outstanding from that wing (another player with an excellent backhand, Ivan Ljubicic, nearly beat Federer in the round robin last week). That’s the forehand side for the lefty Nadal, and we know how good he is with that shot. It’s telling that Federer says he doesn’t like to play Nadal primarily because he’s left-handed.

Oh, one other thing: It helps to play the match of your life (only Nadal didn’t). Last week Nalbandian began to tree in his round-robin match against Ljubicic and never came down.

But this was more than just a good tennis match. It was a moment every sports fan savors—the fall of the invincible. Yesterday, as he found his legs in the fifth and began his improbable/inevitable comeback, Federer, who had won 35 straight matches and 24 straight finals, started to seem like tennis’ version of Duke basketball, USC football, the Kobe-Shaq Lakers, the Yankees, and Tiger Woods. These are the champions we love to hate. Not only do they always win—or at least always seem to win—they do it dramatically, at the last second, with a confidence that seems, infuriatingly, more than human. Gods are impressive, but what’s the point of rooting for them?

The last tennis player with that aura was Serena Williams. It goes without saying that Federer has handled his run of invincibility with more class and humility. Look no further than his post-match words for Nalbandian: “He totally deserved to win tonight.” Still, no great champion can escape a touch of smugness, and it has crept into Federer’s attitude. This is a guy who has denied he has any legitimate rival, characterized Safin and Gasquet as “a little lucky” to have beaten him this year, and has said that his favorite player to watch is himself. By hiring IMG to help market him, Federer may also be getting a new coat of glitz. (In case you didn’t know, he’s now one of the “Sexiest Men Alive,” according to someone.) As the match slid away from him yesterday, Federer grew petulant and disbelieving. At one point he threw his arm up in anger at Nalbandian for daring to question whether one of his glorious forehands had hit the line.

But Federer’s dominance is good for tennis. He brings excellence to the sport when he wins, drama when he doesn’t. His loss yesterday was more than dramatic; it was a relief. Like Duke, the Lakers, Tiger, and the Yankees (but not USC yet, dammit), Federer is human, at least for the moment. He can screw up. I can root for him again—until he wins another 35 straight.