BEIJING -- Cheering from the pool deck, Michael Phelps won his record eighth gold medal of the Beijing Games to become the grandest of Olympic champions.

Jason Lezak held on to the lead Phelps gave him, anchoring the United States to a world record in the 4x100-meter medley relay against an Australian team that did its best to spoil history.

US swimmer Michael Phelps swims during the men's 4 x 100m medley relay swimming final at the National Aquatics Center during the 2008 Beijing Olympic Games in Beijing on August 17, 2008. Michael Phelps became the first man to win eight gold medals at the same Olympics when the US won the men's 4x100m medley relay final in a new world record time. [Agencies]

But Phelps, with a big hand from three teammates, would not be denied. He eclipsed Mark Spitz's seven-gold performance at the 1972 Munich Games.

Even though the Americans have never lost the medley relay at the Olympics, the latest gold was hardly a breeze. When Phelps dove into the water for the butterfly -- the third of four legs -- the Americans were third behind Japan and Australia.

But Phelps, swimming the same distance and stroke that he used to win his seventh gold a day earlier, powered back to the front on his return lap, passing off to Lezak with the Americans in front.

A combination photo shows Michael Phelps of the US holding each of his eight gold medals in the swimming competition at the Beijing 2008 Olympic Games. Phelps won his eighth gold medal on Sunday, beating Mark Spitz's record of seven gold medals won at a single Games in 1972. The medals were won in: (Top row L-R) 400m individual medley (August 10), 4x100m freestyle relay (August 11), 200m freestyle (August 12), 200m butterfly (August 13). (Bottom row L-R) 4x200m freestyle relay (August 13), 200m individual medley (August 15), 100m butterfly (August 16), and 4x100m medley relay (August 17). [Agencies]

Australia's Eamon Sullivan tried to chase Lezak down and appeared to be gaining as they came to the wall. But Lezak touched in 3 minutes, 29.34 seconds -- Phelps' seventh world record.

The Aussies took silver in 3:30.04, also under the old world record, while Japan held on for the bronze.

Earlier, Britta Steffen nipped 41-year-old Dara Torres by 0.01 seconds to win the women's 50 freestyle.

Steffen finished in 24.06 seconds, bettering the Olympic record of 24.13 by Inge de Bruijn of the Netherlands set eight years ago in Sydney. Steffen also won the 100 free.

Torres earned the silver in 24.07 for her 11th career medal in her fifth Olympics, a record for an American swimmer.

Cate Campbell, a 16-year-old Australian, took the bronze in 24.17, and Libby Trickett, the 100 silver medalist, was fourth in 24.25.

Ous Mellouli won Tunisia's first Olympic swimming gold, denying Grant Hackett's bid for a third consecutive title in the 1,500 freestyle.

Mellouli held off Hackett in the closing meters of the grueling race, touching in 14:40.84. Hackett earned the silver in 14:41.53, well off his 7-year-old world record.

Ryan Cochrane of Canada took the bronze in 14:42.69.

Mellouli, who trains in Southern California, is coming off a suspension after testing positive for amphetamines at a meet in November 2006.

Torres, the oldest American swimmer ever, smiled, her head dropping back, when she saw her time.

Still, Torres' second-place finish was a remarkable showing considering she had retired a second time after the 2000 Sydney Games, then got the urge to compete again after having her first child two years ago. Not content swimming in the old-timers division, she set out to prove that age is only a number.

Torres got off to a good start and appeared to be leading midway through the race, a frenetic sprint from one end of the pool to the other.

As they came to the wall, Torres and Steffen were stroke for stroke. The German reached out with her left hand and Torres stretched with her right. Steffen's fingertip got there first.

Australia defeated the United States to win the women's 4x100 medley relay in a world-record time.

Emily Seebohm, Leisel Jones, Jess Schipper and Libby Trickett touched in 3:52.69, lowering the old mark of 3:55.74 set by Australia at last year's world championships in Melbourne. The Aussies won for the second consecutive Olympics.

The US team of Natalie Coughlin, Rebecca Soni, Christine Magnuson and Torres earned the silver in 3:53.30. China's team of Zhao Jing, Sun Ye, Zhou Yafei and Pang Jiaying took the bronze in 3:56.11.

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