KEY BISCAYNE, Fla.—Just how much room is there at the top of the game, especially for a guy who likes to parade his guns in a sleeveless shirt, bangs out the big serves and forehands, and stands a good five inches taller than Andy Roddick?

I suppose it’s a rhetorical question, because if 6-7 Juan Martin del Potro can sustain the level he often reached today while pasting up a three-set upset of Rafael Nadal in the Sony Ericsson Open quarterfinals, he’ll make his own room, even if it means shoving a couple of other guys – Andy Roddick? Nikolay Davydenko? – into the abyss.

The win was as unlikely as it was resounding – less because of what happened than how it transpired. Del Potro won the first set, 6-4. Nadal took the second, 6-3, and led by two breaks (3-0) in the third when the wheels fell off the most reliable wagon in tennis these days. “It was amazing disaster,” Nadal said later, adding an emphatic, “Yeah.”

In tennis, one man’s disaster is another’s Woodstock. With his prospects suddenly looking hopeless, del Potro allowed himself to relax and enjoy the festive atmosphere. They sometimes call this tournament, played in the heart of a region loaded with Latin expats, emigrants, and tourists, the Grand Slam of South America. Today, it was the major of Argentina. That wasn’t North Carolina Tar Heel blue you saw everywhere you looked in the stadium – it was the national colors of Argentina.

Here’s how del Potro described the moment that turned the match his way: “Well, (at 0-3) I start to enjoy the match, the people, the crowds, the quarterfinals.  And I start to play more relaxed and more aggressive, too. Then the match going different.  I broke his serve in 3-0, and then 3-2.  When I was 5-4, I say, ‘Now I can beat him’.  That's was a big difference in my mind.”

Still, this was Nadal, not David Nalbandian, across the net. And that all but guaranteed that things would get more rather than less interesting. Nadal hung in – barely, most of the time – as del Potro cracked the big serve and forehand time and again, exploiting the most glaring of Nadal’s deficiencies – his inability to keep the ball closer to the baseline than the service line in del Potro’s court. Lines were painted there to be hit, not to avoid.

The mandate to hit the ball deep is an easy one to overlook, but a deadly one to ignore or forget. In this case, Nadal’s short groundies ensured that the ball sat up beautifully for del Potro’s ultra-solid strokes. Essentially, Nadal teed it up, and once del Potro straightened out his swing, he was hitting the long drives square and sweet.

It was a struggle for Nadal just to get into the tiebreaker. He had to stare down three match points, and he did it with his trademark courage – twice with an ace; the one that wiped away match point No. 3 was a 131 mph screamer, the fastest serve he hit today. That’s Nadal: always leave room for dessert.

Early in the tiebreaker, it looked as if Nadal had wormed his way off the hook. He scored a mini-break to lead, 3-2, with two serves to come. But he followed it up by surrendering both his serves. He never led again, and del Potro finished him off, 7-3.

Del Potro is talented and surprisingly smooth for such a big man. He’s been an emerging force in the men’s game since the summer of 2008, when he amassed (among other things) a 23-match winning streak and four titles in succession, two on clay and two on hard courts similar to those here in Key Biscayne. His most ardent advocates include John McEnroe. But del Potro had never come within an “hola” of Nadal in four previous matches (Nadal had won nine straight sets against the lanky 20-year old), so it’s fair to ask if Nadal brought his A-game to the park today. His own answer was a definitive “No.”

“I think I played really bad all the time,” Nadal said. “That's the [truth] of the match.”

Moreover, when Nadal was asked how well del Potro played, his reply was uncharacteristically niggardly. He said, “ I think he didn't play one of his best matches, no? That's what I feel. I don't know. You can ask him later maybe. But I think he played well. His level is 7 of the world, no, so he's very good. I think he didn't play an unbelievable match, but I didn't play on my best level today.”

If we didn’t know Nadal better, we might say this was a case of sour grapes. And it suggested that all is not well in Nadal’s world – a suspicion he more or less confirmed at the very end of his press conference, when he was asked if he had a good handle on why he’d played such a ragged match.

“Always is a reason because you are not playing at your level during the tournament. No, I am calm. I am happy about myself, about everything this year, yeah. . .  I don't know. Always is a reason, but it's personal.”

There is probably more to this comment than meets the eye, but we’ll have to wait and see. Meanwhile, del Potro has moved a little closer to the top of the game, clinching the No. 5 spot in the wold rankings.

Those thuds you hear are the sound of Andy Roddick (formerly No. 6) and Nikolay Davydenko (last week’s No. 5) hitting the ground.

Peter Bodo, a senior editor for TENNIS magazine, also writes the TennisWorld blog.