What was Novak Djokovic thinking?

It was a tennis shot heard around the world: Novak Djokovic's forehand return winner to save match point against Roger Federer in the U.S. Open semifinals last year.

So sizzling, so spectacular and so stunning that nearly 12 months on, we're still shaking our heads. Was it lucky or was it good?

The ball flew off the Serb's racket and whizzed across the court, landing right on the line. Not to mention inside his opponent's head.

So, was it lucky or was it good?

"Are you kidding me?" demanded a stung Federer after the match. "I mean, please. Look, some players grow up and play like that. I remember losing junior matches, just being down 5-2 in the third and they all just start slapping shots. It all goes in for some reason, because that's kind of the way they grew up playing when they were down."

It just happens that Djokovic had saved two match points against Federer at the same stage a year earlier, also with forehand winners. "I was just closing my eyes on the match points and just hitting forehands fast as I could," Djokovic said afterward.

Now he had done it again. "I closed my eyes and went for it," Djokovic repeated, having gone on and saved a second match point when Federer clipped the net.

How deep did it pierce into Federer's psyche? Did he toss and turn and wake up in the middle of the night, jumping up and screaming, "Are you kidding me?" We can only wonder. "Go back to sleep, dear," Mirka might have muttered sleepily, woken yet again. "It was just a lucky shot."

All we know is that he was still thinking about it while watching Djokovic play Andy Murray in the fifth set of the Australian Open semifinals this year.

"5-5, 15-40, 25-shot rally. It was a joke how Novak got out of that one again," Federer said.

Not quite. It was 30-40, and a 29-shot rally. But yes, Djokovic hit another searing forehand, this time up the line to win the point.

Hmm. Lucky or good? "In a crazy way for Novak again, he was able to bring it again when he needed it the most," Federer conceded.

And we know that serving for the match against Djokovic in the Wimbledon semifinals two months ago, he was still thinking about it. At 30-30, with Djokovic again returning from the deuce side, like that U.S. Open point, Federer again served to the forehand. Once again, Djokovic swung out, and the forehand went rocketing up the line and landed right next to it -- but just out. Or was it?

Djokovic challenged, and HawkEye showed the ball had been on the line. He had done it again. Only this time, the point had to be replayed because Federer had reached the ball when the call came. For Federer, avoiding a U.S. Open flashback was nearly impossible.

"I simply told myself, it's not possible that he can play this ball again like that. And if so, I would have had tremendous respect for him," Federer said in an interview translated from Swiss-German with sponsor Credit-Suisse. "Usually in some situations you serve down the middle, but I simply wanted to see if he could do it again. In that sense, it was a bit of a game that I was playing with him."

Federer again served to the forehand, and this time Djokovic missed. Match point. "Then I told myself, now you want to do it again," Federer said. "And once again I served to the forehand. I didn't serve particularly well, and Djokovic easily got to the ball. But then surprisingly, he missed the return."

So what about that U.S. Open winner? After winning the match and going on to capture another Wimbledon, Federer seemed to have put the question behind him. "Unlucky, Djokovic played well -- whatever you want to call it," he said.

And before last week's Cincinnati final against Djokovic, Federer insisted he wouldn't be thinking about that semifinal at Flushing Meadows. Perhaps he was right -- serving at 5-5 in the second-set tiebreaker, with Djokovic again making a strong charge back, Federer went to the backhand this time.

But for others, it is still a burning question. Lucky or good? Some feel Djokovic had a solid opportunity to go for the ball off a short 100-mph serve. Others point to how close the shot was to being out. In the end, it comes down to split-second timing. The way he hit it was surely skill, that it stayed in was perhaps chance.

Ironically enough, Federer has hit a famous forehand like that himself -- the inside-out winner hit off Tommy Haas' return, down two sets and break point in the third at the 2009 French Open. Federer went on to win the match and the tournament. What's more, he has said that one of the reasons for his resurgence over the past year is deciding to go for his shots at important moments.

Seeing Djokovic's return go by him may have been the wake-up call, since it was after that match he began to wonder why he was losing so many close contests.

Djokovic has continued to add to his reputation for pulling out daring shots down match point, like he did again in the French Open quarterfinals against Jo-Wilfried Tsonga. Yet even he himself isn't sure how it works. "The one that mentally, I think, pushes more maybe in some moments and obviously gets also a bit lucky and gets the win, you know, that's how it goes," he said.

It's clear, however, that his memorable return winner has become a touchstone for the players, who have repeatedly referred to it when talking about make-or-break shots. "The shorter the sample, the more luck comes into play. In one tournament, yeah, there can be luck," said Bob Bryan. "You saw Djokovic's, that winning forehand that he hit against Fed. He hit it on the line. That's a little bit lucky. One inch and he doesn't hold up the trophy and maybe there's a different No. 1."

"It's like when you talk about the return of Djokovic in semifinals of U.S. Open last year against Federer. You are brave to do it that, yes, or you are crazy," Nadal said.

Good or lucky, it was certainly memorable. And boy, was it audacious.