Michael Phelps may be an 18-time Olympic gold medal winner in swimming, but he is an amateur when it comes to crowd surfing, as shown in this video hosted by TMZ.

In the 42-second clip, Phelps first pumps up the crowd at the XS nightclub in Las Vegas and then gets into an inflatable raft in his attempt to crowd surf.

But seconds later the crowd-surfing try fails, with Phelps falling on his backside and flipping over into a crowd of shouting XS patrons.

The partygoers act as if nothing happened and continue to throw their hands in the air and fist-pump.

Phelps’ attempt at crowd surfing was hardly comparable with the success he had at the London 2012 Olympic Games, when the swimmer beat expectations and won four gold medals and two silvers. The six medals added to his already impressive tally of medals won in the 2000, 2004, and 2008 Olympic Games. His 18 gold medals -- and 22 overall medals -- make him the most decorated Olympian in the modern games.

The crowd-surfing video surfaced a few days after Phelps was interviewed by Glamour magazine about his sister Whitney running in the New York City Marathon on Nov. 4.

While Whitney is competing in the marathon, Phelps’ other sister Hilary is taking part in the related Dash to the Finish Line 5K run the same weekend.

Phelps said he can’t wait to cheer on his sisters.

“It’s something I enjoy, being able to be there for my family and my sisters. I love being able to watch them get out there and have fun and try to accomplish their goals,” the Baltimore native told Glamour.

The Olympian also gave advice to budding swimmers, telling them they should put their personal satisfaction above results. Phelps said listening to music helps him get amped up for a race.

“Just have fun, that’s the biggest thing,” Phelps said. “As long as you’re smiling and having fun, that’s all that matters.”