Germany's Jan Frodeno, who only took up triathlon because a girl he fancied did it, won the Olympic gold medal yesterday in a dramatic sprint over the last few metres.

Sydney gold medallist Simon Whitfield, of Canada, won the silver in the baking hot Beijing swim-bike-run event, and New Zealand's Bevan Docherty the bronze.

Frodeno, a former swimmer and surf-lifesaver who took up triathlon eight years ago, jostled hard with Docherty and Spain's Javier Gomez, both heavily tipped for gold, for front position in the last two run laps.

But the lanky German strode ahead at the final bend, overtaking all of them and leaving a crushed-looking Gomez in fourth place, a full 20 seconds behind his finish time of one hour, 48 minutes and 53 seconds.

"My advantage was that I wasn't one of the big favourites. They had all the media attention," said Frodeno, after punching the air triumphantly.

"I knew this was going to be a tough race, the 10km run was going to be the hardest of my life, but I just realised this is what I have been dreaming of, this is my only chance," he said.

His gold medal surprise came a day after his 27th birthday.

Gomez, who has dominated men's triathlon this year despite an abnormal heart valve that kept him out of competition for several years, said he tired himself out when he bolted ahead and ran the first two laps in a spectacularly fast 14 mins 10 secs to make up for lagging badly coming out of the bike transition.

"I just had a not very good day on the run," said Gomez, who is dating a German triathlete in Monday's women's race.

"I got tired. I did train well but today there were three athletes better than me. I can only say congratulations to them and I will try again in four years," he told Reuters.