He won the shot put today while training partner Adam Nelson took second for a 1-2 U.S. finish on the opening night of the world track and field championships.

Hoffa, who once donned a mask at a meet and called himself "the unknown shot putter," gave the United States its sixth world shot put title in the last seven championships.

"Just being an American shot putter there's a lot of pressure, a lot of expectations," he said. "For me to finally win my first outdoor championship, I don't think I could have picked a better setting. I think I was just ready."

The hot and humid night at Nagai Stadium ended with a remarkable victory for defending champion Tirunesh Dibaba in the women's 10,000 meters. About three-quarters through the race Dibaba, who at 21 already has four world titles, collided with Ethiopian teammate Mestawat Tufa and fell to the track. She got up and grabbed her stomach in pain.

Dibaba said she was about 30 meters behind the pack, but it appeared to be more like 40.

Somehow, she caught them, then sprinted away with a lap to go. Next she will defend her 5,000 crown.

"It was probably the hardest race for me," she said through an interpreter, "having suffered stomach pains and coming back to win. I almost dropped out of the race, but because I represented my country, I hung in there."

Karen Goucher was a surprise third-place finisher, giving the United States its first world championship medal in the 10,000.

The anticipated Sunday showdown between world record holder Asafa Powell and American challenger Tyson Gay in the 100 moved along predictably with both comfortably advancing to the semifinals.

Powell's second-round victory was the fastest at 10.01 seconds, after he obviously slowed close to the end.

"It was as I expected, nothing else," he said. "All is going according to my plans."

After a false start by someone in his heat — and a second one would bring elimination — Gay was cautiously slow out of the blocks but recovered to win his heat in 10.06.

Earlier, after his first-round run, Gay called it "the fastest track I've ever run on."

Hoffa is known for wild antics, including gnawing at a turkey leg during his victory lap at the U.S. championships. But his celebration in Japan was subdued. He bowed and shook hands with every shot put official before taking a U.S. flag for his victory lap.

"This is the world championships," he said. "A certain level of professionalism has to be there. It's not that I don't have fun or enjoy myself, but basically it's the time and place, and today just wasn't the place."

It was the latest triumph in a life of challenges.

"I kind of had that rough childhood but it did make me stronger," Hoffa said. "And it gave me the will to never give up under any circumstances."

He accidentally set a fire that destroyed his house when he was small, and his mother — who was 13 when she gave birth to him — later gave him up to an orphanage. A short time later he was adopted, and he grew into a successful, driven competitor who always yearned to find his birth mom.

"My mother put me up for adoption when I was 4," he said. "Eighteen years later, I end up finding her on the Internet and we got reunited for Christmas, so it was pretty awesome."

The credit for his success, though, goes to his adoptive father, Stephen Hoffa.

"He never let me give up anything," Hoffa said. "I guess basically he raised a fighter."

Hoffa had the four longest throws of the competition, with a best of 72 feet, 3¾ inches (22.04 meters). Nelson, the defending champion and two-time Olympic silver medalist, took second with his season's best 70-10¾ (21.61).

Nelson, a former Dartmouth football linebacker, succeeded despite going to school full-time working toward a masters degree in business at the University of Virginia.

There were days, he said, that he worked from 5 a.m. to midnight, between school and training. He worked hard enough that, with his trademark emotional pre-throw routine of taking off his warmup shirt and slamming it to the ground, he added another medal to his sizable collection.

"What I found," Nelson said, "is that this sort of structure and this sort of daily challenge really drives me to excellence in every facet of my life."

The unexpected bronze for Goucher in the 10,000 gave the Americans three medals on opening day.