By Simon Evans
The YouTube video, posted by Federer's sponsors Gillette, has gone viral in the past 48 hours with more than a million hits to the official version of the clip.
Asked by Reuters if the clip showed a real serve, Federer said: "Well, there's a lot of the debate at the moment. You know how it is with magicians. They don't tell how their tricks work, you know.
"I don't do it that much, but, yeah, it was shot in one piece and it was ... the guy took a chance. It worked out. I'm happy," he said.
Later, when talking about the clip again, Federer said he had "done it before" but was again asked if the clip showed a real action and he said: "Not saying that. A magician doesn't tell how his tricks work, so ..."
Federer won his second round match at the Cincinnati Masters on Wednesday after his Uzbek opponent Denis Istomin retired with an ankle injury.
(Reporting by Simon Evans; Editing by Ian Ransom)