Rafael Nadal relaxed after winning the 2009 Australian Open. At Wimbledon 2013, he simply fell down on the job. Photographer: Carla Gottgens/Bloomberg News
Rafael Nadal relaxed after winning the 2009 Australian Open. At Wimbledon 2013, he simply fell down on the job. Photographer: Carla Gottgens/Bloomberg News

What happened to Rafael Nadal? How did he lose -- in straight sets! -- in the first round of Wimbledon to the 135th-ranked Steve Darcis?

Every major upset demands an explanation. But the first thing to keep in mind here is that Nadal didn’t do a whole lot better at the tournament last year, losing in the second round to the 100th-ranked Lukas Rosol. (At least Darcis qualified for Wimbledon last year, which is more than Rosol had done the year before knocking off Nadal.)

After the Rosol match, Nadal was out for seven months, nursing the bum knee that has been bothering him on and off for years. (And which a lot of people in tennis predicted would end his career years ago.) He skipped the U.S. Open and the Australian Open, and had to withdraw from the Olympics after being named to carry the Spanish flag in the opening ceremony.

Then came Nadal's ridiculous return. Even Michael Jordan needed a little time to get his game back after his brief sojourn into baseball. Nadal reached the finals of his first tournament back, then went on a crazy tear, taking six of the next seven tournaments he played before winning his eighth French Open earlier this month. Now, just when it seemed Nadal had reasserted his dominance, he does something he has never done before: lose in the opening round of a major.

In his press conference after the match, Nadal refused to blame his knee, not wanting to take anything away from Darcis. But it’s tough to believe that it wasn’t the knee, or a combination of the knee and the grass, which is -- counter-intuitively -- hard on players with knee problems. Grass may be soft, but it’s a difficult surface on which to change directions, and it tends to keep balls low, requiring more bending. That's especially true on soggy days such as today.

So what does this mean? If nothing else, Nadal has lost an opportunity to close the gap with his long-time rival, Roger Federer. Most tennis fans prefer Federer; his footwork is more effortless, his strokes prettier. It’s hard to argue that history will remember Nadal as the greater player, and this is an especially bad time to make that case. But how do you define greatness? Nadal will probably never match Federer’s total of major tournament wins. (Nadal has 12; Federer 17.) Nadal emerged later, has been less consistent and will almost certainly stop playing sooner. As bad as he was today, it's worth remembering that Nadal is a warrior, a player who converts adversity into intensity. When he's at his best, no one -- including Federer -- is better.

(Jonathan Mahler is a Bloomberg View columnist. Follow him on Twitter.)