By Martyn Herman RIO DE JANEIRO, Aug 21 (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. 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. More...