A couple of weeks ago, Paul Annacone — who I find to be the best tennis commentator going — made a fantastic point. He was asked to choose the greatest server in men’s tennis history, a fairly typical tennis question. But he would not answer it. Instead, he insisted, on making a subtle but important distinction.

The greatest SERVE in tennis history, he said, might indeed belong to those players who are always mentioned in such conversations, John Isner or Andy Roddick or Goran Ivanisevic or Ivo Karlovic or going way back to Roscoe Tanner or Pancho Gonzalez. Those (and you can throw others like Milos Raonic or Kevin Anderson) hit the hardest serves. They get the most aces. They go as far as their massive serves can take them.

But, as Annacone said, he sees a difference between best SERVE and best SERVER. That is to say, when you ask who is the greatest server, you have to define terms. Are you asking who hit the ball hardest and is mostly likely to get an ace? Maybe not. What you might really be asking is this: Which player would you count on most to hold serve on the biggest stage and in the biggest moment?

In that discussion, two names rise above. There’s Pete Sampras, who was so famous for his serve that people called him Pistol Pete.

And there’s Roger Federer, whose overall greatness and relatively mild-looking serve might cover up the possibility that he really might be the greatest server in the history of men’s tennis.*

*I keep saying “men’s tennis,” not only because it is its own category but because I don’t think there’s any question at all that Serena Williams has the greatest serve in women’s tennis. And if greatness is about judging an athlete against competitors, well, Williams’ serve is so much better than anyone else’s that I think it has to be No. 1 overall.

Federer has never been anything close to the hardest server. He has never been among the handful of players with the most aces per game. When people talk about Federer’s game, they will often bring up other parts — his forehand, his movement, his net play — before talking about his serve.

And yet when you put it all together — first serve, second serve, angles, consistency, deception — Federer’s serve is like a brilliantly conceived magic trick. Year after year after year after year, he wins more service games than anybody. He has won 5,000 more service games than any player since they started keeping count.

Fed has won his service games 88.9% of the time, a higher percentage than any of the greatest players of the last 30 years including Rafael Nadal, Novak Djokovic, Andy Murray, Andre Agassi or even Pete Sampras. Yes, it’s true, some of those massive servers we mentioned earlier — Karlovic, Isner, Raonic, Roddick — have slightly higher win percentages, but only Roddick among them regularly faced those big moments, grand slam semis and finals against the world’s best players.

And we are pretty clear on Fed vs. Roddick. Federer won their head-to-head matchup 21-3. And in their classic five-set match at Wimbledon, it was Federer who held serve the longest.

I bring all this up because Tuesday night, in his U.S. Open quarterfinal against Grigor Dimitrov, we saw Federer’s magic serve. For one set. Then it was gone.

And it makes you wonder just how much longer we will get to enjoy that magic.

Dimitrov is 28 years old and a fascinating story himself: He grew up in Bulgaria, hardly a professional tennis hotbed — it was basically the Maleeva sisters, Magdalena, Manuela and Katerina — and he idolized Roger Federer. Just about every tennis player of his generation idolized the man, but Dimitrov went further. He made himself into such a Federer clone, so much so that people began calling him “Baby Fed.”

Dimitrov, like Fed, hit a big forehand. Dimitrov, like Fed, hit a one-handed backhand that was particularly bruising when sliced. Dimitrov, like Fed, flew around the court, chasing down everything. Dimitrov, like Fed, hit a big serve.

In fact, Dimitrov hit a bigger first serve than Fed, if you are judging by mph. Before his shoulder injuries, Dimitrov could pump his serve at 135 mph or higher; Fed did not hit his serve that hard.

All of that was good enough to get Dimitrov into the top 20, the top 15, the top 10 … he even reached No. 3 at one point, though that was a somewhat odd time when most of the Top 10 players were injured or in the middle of massive slumps. But he could never quite break through because, to oversimplify, he lacked that Federer magic. Sure, his first serve was bigger, but it wasn’t even close to as good. His forehand was huge, but it wasn’t Federer’s forehand. He moved fast, but never quite as economically as Fed.

The two men became good friends, practice partners, etc. (everybody likes Dimitrov). But their head-to-head match-up before Tuesday night was entirely one sided. Federer had easily won all seven matches they played, dropping just two sets along the way.

And so it began Tuesday on a beautiful evening under an orange moon in New York. Fed may be 38 years old — he was the oldest major quarterfinalist since Jimmy Connors had his legendary old man run at the 1991 U.S. open — but he won the first set in a blur of genius. It was over before it started. Dimitrov could not return Federer’s serve.

This has been the story for almost 20 years. Fed hits that serve and players jab it into the net or send it sailing wide or clank it off the frame of their rackets. When Fed did face a break point, he hit the ace to get out of it. That’s another thing about his serve (and Sampras’). He has another gear when he needs it.

Anyway, it was a breezy 6-3 first set, and Dimitrov looked entirely shaken, and undoubtedly Fed fans began looking ahead. In the semifinals, Fed would face the new bad boy of tennis, Daniil Medvedev, who happily tells the New York crowds that their boos and taunts power him the way revenge powers Inigo Montoya. Win that, and the final would probably feature Fed against the man himself, Rafa Nadal. The two men have never played each other at the U.S. Open. It would be a match for the ages.

Only, something happened after that first set. Well, a couple of things happened. One, Dimitrov settled down and began playing superb tennis. He knew full well that this was the biggest match of his career, and he desperately needed to find a way back in. And he did: He began crushing his ground strokes and imposing his will. For those of us who have been rooting for Dimitrov to take his game higher, it was wonderful to see his fight.

Two, Roger Federer began to age before our very eyes.

And the first sign was his serve. Federer’s serve didn’t LOOK different. He hit it the same speed with what seemed the same angles. But something had changed. Dimitrov began getting the ball back. He broke Federer to take the lead and then had a chance to close out the set. Instead, Dimitrov had an agonizing and nervy game and, in the end, double-faulted away his chance.

Then when everyone expected Federer to put his foot on the gas and put his man away. But instead Federer followed up with one of the worst service games of his entire career. He made four errors. He lost the set.

It was startling to see. And it would get more startling. Federer had enough left to win the third set (again, Dimitrov’s nerves seemed to get the best of him) but by the fourth set he was done. His serve was a pillow toss. His groundstrokes looked half-hearted. He could not move. The ball kept clanking off his frame. By the time the match ended, Federer would make an almost unheard of 61 unforced errors, most of them on his usually invulnerable forehand.

And his serve … just … laid there. Oh, sure, Fed still had enough tricks to hold his serve four times in that fourth set, once in a crazy 20-point marathon where Federer looked like he might just expire, like a parking meter. He tried everything. He charged the net repeatedly. He tried to mix in all sorts of different speeds. He tried to make Dimitrov feel the pressure. But this time Dimitrov held it together, won his decisive service game, and forced a fifth set.

It was clear then that Federer was done. His body had broken down. He did what he never does … he gingerly walked off the court for a medical timeout. You could read the lips of Fed’s agent talking to Fed’s wife Mirka: “Back.”

All the while, Dimitrov ran around the court, did pushups and showed that he had enough left to play all night.

Federer did come back out on the court to play that final set. He knew that he had nothing left. Maybe, in the back of his mind, he began that final set with the hope that his serve would save him one more time and that Dimitrov would melt under the bright lights. When Dimitrov took the 4-0 lead — with two service breaks — there were no illusions left.

Federer absolutely could have retired then. There would have been no shame in it.

But Roger Federer has a code he lives by. He won’t talk about it, but I feel sure he stayed out there not only for the crowd and the principle but, most of all, for Grigor Dimitrov himself. I think Fed knew that if he retired, it would have taken a bit of the joy and pride out of the victory for Dimitrov. It would have dampened Dimitrov’s singular moment. So he played on.

And, even though he was so clearly drained and exhausted and beaten, he somehow held serve. Then, a game later, he somehow held serve again. It was all he had to give.

“How are you feeling?” reporters asked him after Dimitrov held for the decisive 6-2 victory in the final set.

"This is Grigor's moment and not my body's moment,” Fed said. “So, it's okay."

There were more questions to ask but there was no point in asking them because there are no answers. Can Roger Federer at 38 or 39 or 40 hold up to two weeks of pounding tennis? Can he again find his best tennis in three or four consecutive matches against the world’s best? Can he keep winning his serve time after time after time?

We don’t know. The only two things we do know is that sooner or later, time runs out on every great athlete … and all we can do is hope that Roger Federer’s time isn’t up just yet.