Phelps won the 200m butterfly gold in a world record time of 1min 52.03 secs, eclipsing the elite group of Mark Spitz, Carl Lewis, Paavo Nurmi and Larysa Latynina, who have each won nine career golds. Then Phelps led the US to the win in the 4x200m relay, smashing the world record ahead of Russia for the silver and Australia for the Bronze.
Phelps' victory has brought him closer to surpass Spitz's other record of seven golds in a single game.
Prior to the butterfly final, Phelps had won the 400m individual medley, 200m freestyle and was a member of the United States 4x100m relay team that took gold ahead of France in a stunning final.
He still has the 4x100m medley relay, 200m individual medley and 100m butterfly on his program, but has said with the gruelling 400m IM and relay wins, the hardest part of his eight-gold quest is behind him.
Latynina and fellow Soviet gymnast Nikolai Andrianov hold the record for the most medals in all, with 18 for women and 15 for men respectively, but with at least one more Games left in him Phelps could break all record.