London, England (CNN) -- The ball looked real. The shot looked real. Even the fear in the eyes of the hapless assistant, standing in front of one of the most powerful right arms in the history of tennis, looked real.
Yet almost as soon as the "William Tell" video went viral, showing the Swiss tennis star Roger Federer smashing a bottle from someone's head, the blogosphere was rampant with conspiracy theories as to whether the video was faked or not.
"You know how it is with magicians. They don't tell how their tricks work," was all Federer said when asked by Reuters about the legitimacy of the video.
The mystery has helped the promotional clip, shot for Gillette, rack up more than five million hits on YouTube already.
"[The video is] Impressive if true," remarked Dr. Simon Tuplin, an expert in the aerodynamics of sports balls at Loughborough University's Sport Technology Institute, England.
But was it actually real?
"There is no reason, aerodynamically, that I can see from the video, why the shot is not possible," he explained.
"Low speed video footage is notorious for providing false impressions of ball flight due to lens curvature, parallax effects and drop frames. I think in this situation, however, it is simply a case of, is he [Federer] capable of that level of repeatable precision?"
Few would bet against a player like Federer, winner of 16 grand slams and considered one of the best to have ever graced the sport, having the skill to pull off a challenge as audacious as this.
But does it matter if such videos are true are not? Viral video that blurs the boundaries between truth and fiction have had viewers second guessing their veracity for years.
Nike, in particular, has filmed stunts ranging from basketball star Kobe Bryant jumping over a speeding Aston Martin to A.C. Milan and Brazil striker Ronaldinho striking the crossbar from the half way line over and over again.
Yet it is exactly this uncertainty that has fueled the speculation, and the interest, in the films.
"They could be real, they might not be real," laughed Charlie Brooks, a spokesperson for Nike, when asked whether the Nike videos were fake or not. "That's all part of it."
What do you think? Here are four more unbelievable viral videos.
In one of Bryant's other viral videos, he hangs out with the stars of U.S. TV show Jackass before performing a slam-dunk over a pool of snakes. Alas, it looks about as real as some of the kung fu moves from Crouching Tiger, Hidden Dragon.
But this video, of Bryant leaping over a speeding car, is far harder to dismiss.
Andy Murray takes his racket and glum demeanor to the streets in a series of grim looking urban settings. The unnerving accuracy of each of his trick shots gives the film a slightly supernatural air, as if David Blaine had taken up tennis instead of card tricks.
Ok, so we know that Cristiano Ronaldo is fast, but fast enough to beat a Bugatti over 100 meters, and then back again? Really?
One of the most successful sports viral videos of all time, Ronaldinho takes the ball to the half way line and hits the crossbar over and over without the ball touching the ground. It looks real, and if one footballer could do it, Ronaldinho could. Well, Ronaldinho in his Barcelona pomp, rather than the A.C. Milan version....