By Martyn Herman

RIO DE JANEIRO (Reuters) - Peter Sagan's hopes of winning a surprise gold medal in the Olympic mountain bike cross-country race disappeared along with the air escaping from his tyres on Sunday.

The Slovak, a giant of the road where he won the world title last year and claimed a fifth consecutive Tour de France points jersey last month, gambled on returning to his mountain bike passion, rather than race on the road on opening Saturday.

But after waiting until the 15th day of action to try and make his mark on the Games, his challenge was effectively over inside the first 20 minutes of the race.

Starting last of the 49 riders because of his low Union Cycliste Internationale (UCI) ranking in mountain bikes, Sagan sliced through the field and was already in the leading group of five early in the first of seven laps of the muddy 4.8km circuit.

A fascinating and eagerly-anticipated battle between the one-day road specialist and dirt warriors like eventual winner Nino Schurter, Julien Absalon and Jaroslav Kulhavy looked to be brewing in the Rio hills.

Then the all too familiar hiss, the bugbear of recreational cyclists, spelt trouble.

His first puncture cost Sagan two and a half minutes as he rode on for half a lap before reaching the technical zone with his front tire flapping around the rim.

With a new wheel on he tried to make up some ground but another puncture following a small tumble left him simply making up the numbers. At one point he was even forced to carry his bike up a rocky climb.

"In the start I felt great and was in the top three,” the 26-year-old Sagan, who rides for the Tinkoff Pro team, said.

"But then I had a puncture and I had to go for half a lap before I could get a wheel change. Then it was impossible to make contact with the first guys.

"I also had a puncture on the third or fourth lap, only a small one, but my wheel felt like chewing gum.

"Two punctures were very bad for my mentality. I did everything, but it was impossible to get in the top 10. I felt I could make the top 10."

He eventually finished a lap down in 35th place but said he did not regret the experience, even if the mechanical gremlins had deprived him of a real chance.

"After seven years, I am back on the mountain bike, and I was happy to try," said the former world junior champion, who said he would now concentrate his mind back on the road and the world championships coming up in Doha.

(Reporting by Martyn Herman; Editing by Andrew Hay)