!Pic by Pete Bodo

It wasn't so much that Novak Djokovic lost to Andy Murray at the Western and Southern Open in Cincinnati; it was the way he lost the final. There he was, listless and fatigued right from the get-go, struggling with his footwork and strokes, committing tactical errors—none of which was more painful and costly than the volley he smacked right back at Murray while facing break point during one of the most blistering rallies of the match. Murray won the point and went on to take a 4-3 first-set lead, after which Djokovic got but one game.

How can a player look, well, rusty when he's coming off a Masters 1000 win (just the previous week in Montreal; it was Djokovic's fifth Masters title of the year, a record)? When he'd accumulated a 57-1 record for the year and was riding a 31-match hard-court winning streak into the final? Evidently it was a combination of general fatigue and injury, neither of which could have been predicted even hours before the match. After all, the major quantifiable component in Djokovic's magnificent season has been his fitness and endurance.

This is a truly puzzling turn of events, and one with resonances for the upcoming U.S. Open, when you consider that after the match, Djokovic admitted, "I could have maybe played another couple of games, but what for? I cannot beat a player like Murray today with one stroke."

Fair enough. But it's never a good omen when someone quits. In this case, we also saw that massive thunderstorms were imminent when Djokovic pulled the plug down 6-4, 3-0. If Djokovic had wanted to preserve his chance to equal or surpass John McEnroe's record .965 winning percentage of 1984 (complied on the strength of an 82-3 record), or somehow stem the tide and reverse the momentum in the final, he could have hung in there until the rains came and then. . . whatever. But he clearly made up his mind to quit.

The way Djokovic lost to Murray may have occasioned greater rejoicing in the streets of Manacor, Spain or Bottmingen, Switzerland than if he had gone down swinging, 7-5, 6-4, for it points to more troubling potential scenarios for the Serb at the U.S. Open than the typical "any given day" loss would have. Most prominently, Is Djokovic sated, or fried?

In the first half of the year, Djokovic was essentially indefatigable and invincible; he was a doomsday stroking machine, impervious to all frailty of the flesh—a condition he attributed to his training regimen and new, gluten-free diet. If ever a man had a right to be tired, it was Djokovic when he rolled into Madrid in May with a 33-0 record and then won the title over Rafael Nadal; if ever a man had a right to be dead, it was Djokovic the very next week, as he blasted his way through the Rome draw, again overcoming Nadal in the final.

Djokovic had a well-earned rest of about three weeks after he suffered his first loss of 2011 (at the hands of Roger Federer) in the Roland Garros semifinals. But he won his next tournament, Wimbledon, and then had a break of about a month before his triumphant return in Montreal. Given the wall he hit in the final of Cincinnati, you have to wonder if that triumph in Montreal wasn't accomplished with smoke and mirrors, by a guy relying less on enthusiasm than muscle memory, confidence, and reputation. Is Djokovic back-back, or just back?

The heat and humidity in Cincinnati have always been brutal, but for many years now the top ATP pros have negotiated that hazard—thanks mainly to the long break the best players take after Wimbledon. Federer, for example, has almost always found a second wind in North Ameica in August. In the years between 2004 and 2008, the five-year period during which he won each U.S. Open, he missed just one of the 10 Masters tournaments played in Canada and Cincinnati—and won four of the nine he entered. He's always played the second half seeming just as fresh in mind and leg as he was during the first six months.

Nadal has been more prone to struggle on hard courts, and the accumulated fatigue of his routinely exceptional performances on clay have often been thought partly responsible for his failure to win the U.S. Open, until 2010. Yet whatever degree of fatigue Nadal felt, he always prepared for the Open diligently; he's played both summer Masters events every year since he qualified for the main draw at either (he's won two titles in that 14-tournament string).

Djokovic certainly has been doing yeoman's work this year, but Federer and Nadal have both had a comparable number of matches up to this point, even if they were slightly less successful even during their greatest years. Thus, Djokovic's attitude and actions are a little more mystifying than it may seem, if you just look at his degree of success. There's no real reason why he couldn't or shouldn't have been able to play through the two hard-court Masters, performing the due diligence of all champions.

The most logical explanation for Djokovic's swoon in Cincinnati is that he was just trying to take precautions in advance of the U.S. Open, loath to risk further injury to his shoulder. But we know that horses and even dogs can literally run themsleves to death, or to such a point of exhaustion that they will never recover to be the creatures they formerly were. Can that happen to a tennis player? I doubt it—at least they are unlikely to burn out permanently. But it's also true that there's a point where the spirit may be willing but the flesh is not.

Djokovic mimimized the issue of overall fatigue—hail, let's not be all polite about this, just call it  "burnout"—after the match, telling the press: "I was generally exhausted playing many matches, but the exhaustion is not the reason (I quit). The reason is shoulder pain. I just could not serve."

This threat of a shoulder injury is a more insidious one when it comes to Djokovic's chances at the U.S. Open, but burnout can't be discounted. And the skeptic in me wonders just how badly he's hurt, given that Djokovic freely admitted that he could have played on against Murray. And looking forward, he commented: "The good thing is that there is a week, eight days [before] the start of the U.S. Open. So I think that's enough time for me to get ready."

Those don't sound like the words of a man who's seriously worried about missing the big show.

Given that neither Nadal, Federer, nor Murray has been lighting it up these days, the questions raised by the way Djokovic abandoned the Cincy final are apt to provide those rivals with some inspiration—motivation that may mean nothing, should Djokovic revert to the machine he was as few as two weeks ago. But if he's hit a wall, don't expect that trio to stand around wringing their hands. Nadal in particular may relish a final chance to collect a bit of Grand Slam payback, on the court he'll step onto as the defending champion in just a one week's time.