Phelps splashes out on a record.
Phelps collected his fourth gold in Beijing in the 200 metres butterfly and made it five out of five just under an hour later in the 4x200m freestyle relay, both in world record times.
Added to his six golds from Athens four years ago, the two victories saw Phelps surpass the nine golds won by Paavo Nurmi, Carl Lewis, Mark Spitz and Larysa Latynina to cement his place in Olympic history.
Phelps, who remains on course to surpass Spitz's achievement of seven golds in one Games, said: "I am almost at a loss for words.
To be the most decorated Olympian of all time, it just sounds weird. I am speechless.
"It started to sink in a little after the butterfly. I was trying to focus on my next race but I kept thinking 'Wow! Greatest Olympian of all time."
Phelps, 23, led from the halfway point in the butterfly but was made to battle hard to win in a world record time of 1:52.03 seconds, beating his own previous best by 0.06s ahead of Hungary's Laszlo Cseh and Japan's Takeshi Matsuda.
Phelps, who remains on target to surpass Spitz's achievement of seven golds in one Games, said: "I could not see anything for the last 100m, my goggles filled up with water.
"I wanted to break the record and wanted to go 1:51 or better, but in the circumstances I guess it is not too bad."
As expected, Phelps then led off a dominant American quartet in the 4x200m relay, swimming the first leg more than two seconds under world record schedule. His team-mates Ryan Lochte, Ricky Berens and Peter Vanderkaay carried on where he left off and clocked a new world record time of 6:58.56, the first time the seven-minute barrier has been broken.
In a thrilling battle for the other medals, Russia claimed silver ahead of Australia, with Britain's quartet of David Carry, Andrew Hunter, Robert Renwick and Ross Davenport setting a new British record of 7:05.92 in sixth.
Another world record was broken in the women's 200m freestyle, Italian Federica Pellegrini taking gold in a time of 1:54.82 to lower her own mark set earlier in the week by 0.63s.
Slovenia's Sara Isakovic claimed the silver ahead of home favourite Jiaying Pang with Britain's Caitlin McClatchey sixth in a time of 1:57.65.
In the women's 200m butterfly, British pair Jemma Lowe and Ellen Gandy failed to qualify for tomorrow's final.
In the men's 200m breaststroke, Britain's Kristopher Gilchrist and Ireland's Andrew Bree both failed to qualify for tomorrow's final.
SIMON TERRY'S hopes of a medal in the men's individual archery ended after a disappointing final end in his tie with Finland's Matti Hatava.
Terry, who was seeded seven, had led the contest since the second arrow and led 81-77 going into the last end, but scores of eight, eight and seven allowed the Finn to nip in and win 105-104.
Team-mate Laurence Godfrey was also knocked out in the first round, beaten 114- 109 by Bair Badenov of Russia.
WINSTON Gordon's bid for gold in the men's under-90kg judo came unstuck in the preliminary round when he was beaten by Uzbekistan's Khurshid Nabiev.
He must now wait to see how Nabiev progresses to determine if he earns a repechage spot.
BRITISH interest in the fencing came to an end after Richard Kruse was beaten - but he did at least record the solitary British win in Beijing.
Kruse, who reached the quarter-finals of the individual foil four years ago, beat Romania's Virgil Salscan in his first contest but was then beaten 10-9 by Germany's Peter Joppich in his last 16 bout.
BRITISH synchronised three-metre springboard pair Nicholas Robinson-Baker and Ben Swain finished seventh out of eight in their final at the Water Cube in B eijing.
GOLDEN BOYS: The triumphant US 4x200 metres relay squad of (left to right) Ricky Berens, Ryan Lochtie, Peter Vanderkaay and Michael Phelps