Phelps out-touched runner-up Serbian Milorad Cavic by merely 0.01 second with an Olympic record time of 50.58 seconds. Australian Andrew Lauterstein got the bronze in 51.12. World record holder Ian Crocker finished the fourth in 51.13.

The unstoppable Phelps removed a major obstacle on his way to an unprecedented feat of eight golds at a single Olympics.

Phelps stood upon the deck as the defending Olympic champion. Taking off in lane five, he was about half a body length behind Cavic in the first lap, and was the seventh to touch wall in the first 50 meters. But the 23-year-old superfish splashed ahead in the last 50 meters and won amid jittery cheers of the spectators.

"Beforehand, Bob (his coach) said it would be good for me if I lost. When he said that I was fired up. I said, 'I'm going to go for it'. And when I saw that finish I said 'wow'," Phelps told reporters after the race.

"When I took that last stroke I thought I lost the race there, but it turns out that was the difference. I'm just lost for words," he said.

Phelps again screamed after the victory and punched hard at the water. He was so overjoyed that he barely noticed Cavic congratulating him in the pool.

"I feel a little bit of everything - relief, excitement, everything. I had to take my goggles off first to make sure the one was next to my name," he said.

Phelps has one more race in the 4X100m medley relay to go before leaving Beijing.