On Labor Day evening at the U.S. Open, in a fourth-round match that was moved from the grandeur of Arthur Ashe Stadium to the slightly-more-intimate Louis Armstrong Stadium due to a rain delay, 31-year-old Tommy Robredo stunned the world by taking out 32-year-old Roger Federer in straight sets.

To say it was unexpected would be an understatement. Most fans and journalists viewed this match as a mere formality standing in the way of the much-anticipated Federer-Nadal quarterfinal. After all, prior to this match, Robredo and Federer had faced off 10 times in their careers. Federer had won all 10 meetings, and they hadn't even been close.

Plus, Federer had seemed to finally be in good form for the first time all summer, cruising through his first three rounds by barely dropping a set. Robredo was almost pushed to a fifth set by a qualifier in the third round, and seemed injured in the process. It all seemed pretty straight-forward.

But let's back up a little bit.

Most everyone knows Federer. He's one of the best players of all time, a legend, an icon of sport. He's a dorky father, a loving husband, a stylish spokesperson and, oh yes, a really nice guy. He has 17 Grand Slams to his name, over 900 wins, and has won over $77 million -- and that's just on the court. Anna Wintour comes to watch him play -- even when he's moved to Armstrong.

Far fewer people are familiar with Robredo. The often-overshadowed Spaniard has been on tour for as long as Federer has -- 15 years, which is a lifetime and a half for a tennis player. He's been ranked as high as No. 5 in the world, has a solid but not spectacular game, and he feels most at home on clay courts.

He's a bit of an odd, albeit endearing, character. His parents named him after Tommy by The Who. He used to sport long, wavy hair that inspired fans to start referring to him as "Disco Tommy." His clothes, well, they often don't match, and it's likely that Anna Wintour had never heard of him until Monday night.

A year ago, Robredo and Federer couldn't have been in more different places in their careers. Federer had just regained the No. 1 ranking, and he came into the Open riding high, having had just won Wimbledon and the Western & Southern Open Masters in Cincinnati that summer. He was, as it seemed he always had been and would remain, on top of the world.

Robredo, meanwhile, was just returning to tour after an eight-month layoff due to a career-threatening leg surgery. He was ranked well outside the top 200, and testing the comeback waters in Challengers -- the minor leagues of the tennis tour. Far away from the glitz and glamour associated with the top echelons of the game, he was out of sight and therefore out the minds of most tennis fans. In fact, most had probably assumed he'd retired.

But in 2013, Robredo has experienced a resurgence. With a renewed passion for the game, he has won two titles this year and climbed back to No. 22 in the rankings. His most remarkable feat was at the French Open this year, where he made the quarterfinals after being the first player in the Open era to come back from two sets down in three straight matches.

As Robredo has been leapfrogging up the rankings, Federer has been slowly and incrementally heading down them, from No. 1 to No. 7. Though he has been in a well-publicized decline all year, this summer has been particularly alarming, as he had a bad back, fiddled with the size of his racket, and lost to players ranked outside of the top 50 in three straight tournaments.

It seems dismissive and overdramatic to say that he's not "Federer" anymore, but it's accurate and understandable to say that Federer at 32 is not the same player he once was, at least not day in and day out. The aura of invincibility that he used to have is, if not completely gone, much harder to spot than it used to be.

And still, the two veterans entered their fourth-round encounter on Monday under the billing of a champion and an afterthought. Those narratives, they sure can be blinding.

The first set was a messy, back-and-forth affair. Robredo got the break early, but Federer found his way right back in the match. Then Robredo, shockingly, broke Federer again and was poised to serve for the first set, but was broken as he was serving for it.

When they entered a first-set tiebreaker, it seemed as if the rest of the match had already been scripted: Federer would find the goods when he needed to. Robredo wouldn't. Rinse and repeat.

After all, you'd think that after 302 weeks at No. 1 and 77 titles, confidence in key moments against a by-all-accounts-inferior opponent would be a given.

But that's just not how it works. That confidence, it's a fleeting thing.

"[O]bviously Roger, when he was No. 1, to the Roger right now, he's not maybe with the same confidence," Robredo told reporters after the match. "Obviously he's the same player and he plays unbelievable. But I knew that if right now I had a little bit more chances, maybe he will had a little bit of doubt, no?"

"Yeah, probably," Federer agreed. "Confidence does all these things. It takes care of all the things you don't usually think about, you know."

And so, from the tiebreaker on, confidence took over -- only it had a different host than it usually did. On the important points, Federer's forehand sprayed, his serve was unreliable, and his backhand found the net.

Robredo, however, was steady as a rock, and managed to come up with big winners when he needed to.

Federer was 2-14 on break point. Robredo was 4-7. Therefore, although Robredo only won nine more points than Federer did, he ran away with the match 7-6, 6-3, 6-4.

For Robredo, it was a triumph -- one of the biggest wins of his career. He'll face his compatriot Rafael Nadal in the quarterfinals, but no matter what happens, he'll be in the top 20 after this tournament -- a far cry from the No. 178 next to his name a year ago.

"For me, Roger for the moment is the best player of all times," Robredo said. "And to beat him in a huge stadium like the US Open and in a Grand Slam, a match of five sets, it's like a dream, no? I am so, so happy."

For Federer, it was another sobering moment in a year chock-full of them. His uncomfortable search for answers continues, as does his slide.

"I kind of self-destructed, which is very disappointing," he said. " I kind of feel like I beat myself, you know, without taking any credit away from Tommy."

There's nothing quite as rewarding as watching a veteran pull it all together for one last push toward greatness. There's nothing quite as unsettling as watching a legend come to face-to-face with his career's mortality. There's nothing quite as confusing as seeing both happen at the same time.

It's hard to know what to make of a match like this, one so lopsided and against-type. It's hard to know how much of it was real and how much was a mirage.

It was two careers, both nearing their end, passing in the night.

It was depressing. It was inspiring. It was tennis. 

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Lindsay Gibbs is a freelance writer, tennis lover, and author of "Titanic: The Tennis Story." She is a co-founder of The Changeover and can always be found @linzsports.