American Reese Hoffa shed forever the tag of the unknown shot putter when he hurled his way to a gold medal on the opening day of the World Championships.

The world indoor champion, who used to wear a mask to emphasise his position as his country's least known shot putter, fired 22.04 metres with the third of his six throws to push defending champion Adam Nelson down to second place.

''For me to win my first outdoor championship, I guess I couldn't have picked a better setting,'' said the 29-year-old.

Hoffa was put in a children's home by his mother after burning down his family home when he was four and was later adopted.

He was reunited with his birth mother two years ago after tracking her down on the internet but thinks his difficult early years contributed to his success.

''Ultimately, I kind of had that difficult childhood, but it made me stronger, it gave me the will to never give up,'' he said.

Nelson achieved his best distance of the year with 21.61 metres on his second throw but blew up with a string of four fouls as he tried in vain to better Hoffa.

''Reese flat out beat me tonight, 22.04 in a major championship is an unbelievable accomplishment,'' said the 32-year-old, who won his third world championship silver.

Reese's opening 21.81 would have been enough for gold but having thrown 22.43 in London [Images] earlier this month, he knew he could do better.

His third throw was the only one of the evening to sail beyond the 22-metre tape and he celebrated with a roar of a approval, his arms raised in the air with delight.

After fouling his final throw, Nelson led the cheers for his one-time training partner and Hoffa paid tribute to the contribution the 2005 champion had made to his career .

''Adam showed me the ropes,'' he said. ''For many years he kicked my butt in practice to make sure I was physically and mentally ready.'' Andrei Mikhnevich of Belarus, who won the 2003 world title after serving a two-year ban for failing a dope test at the 2001 championships, took the bronze with a throw of 21.27.

It was sixth men's shot put gold in the last seven world championships for the United States with Hoffa following three-times champion John Godina, CJ Hunter and Nelson in topping the podium.

''The tradition goes on,'' said Hoffa. ''I think it's because in the United States, even just to make the team, you really have to earn your way there.''