Showing no sign of fatigue or weariness after his gruelling schedule, Phelps cruised through the preliminary round of the 100 metres butterfly when he won his heat in 51.80 seconds at the U.S. Olympic trials.
Phelps has already qualified for London in three individual events and remains on course to add two more, the 100 butterfly and the 200 individual medley, which will be decided on Saturday night.
"I was happy with that, I think it's probably one of my fastest morning heats ever," he told reporters.
"The biggest thing was to make sure I got in the second (semi). It gives me an extra three minutes to recover after the IM and every minute helps."
Colorado teenager Missy Franklin stayed on track to swim seven events in London after topping the qualifiers in the women's 200 backstroke.
The 17-year-old stopped the clock at two minutes 08.35 seconds, more than half a second ahead of her closest rival. She will swim in the 100 freestyle final on Saturday night, which doubles as a chance to make the 4x100 freestyle relay.
Anthony Ervin, the 31-year-old 2000 Sydney Olympics champion, outsprinted his younger opponents to post the quickest time in the 50 freestyle heats at 21.83, while Kate Ziegler was fastest in the women's 800 freestyle heats in 8:027.61.
Katie Hoff, who won five events at the 2008 U.S. trials but was unable to win a gold medal in Beijing, failed in her last chance to make her third Olympics team when she finished 13th after struggling all week with a virus.
"Obviously I wanted to do better but I gave it my best shot," Hoff said.
"I'm actually proud of myself for doing it. Somebody told me my career is about not rolling over in adversity. I don't want to be that girl."
The 800 heats also marked the end of the brief comeback of former Olympic champion and world record holder Janet Evans.
The 40-year-old mother of two retired after the 1996 Atlanta Games but decided to have one last shot at making another Olympics, although she knew she had little hope, finishing 53rd in the preliminaries.
"This became more for me than just trying to make the Olympics," Evans said.
"I could have slept in every day and played with my kids but this was just about trying something different and I'm so proud of myself for getting this far.
"But this is definitely it. I just signed my retirement papers."