BEIJING — Cheering from the pool deck, Michael Phelps won his record eighth gold medal of the Beijing Games on Sunday 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 400-meter medley relay against an Australian team that did its best to spoil history.

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, an iconic performance that was surpassed by a swimmer fitting of this generation: a 23-year-old from Baltimore who loves hip-hop music and texting with his buddies.

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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.

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 in his personal Great Haul of China.

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