Phelps, who established the previous record in winning gold in Athens four years ago, clocked 4min 07.82sec, an easy swim for the man who lowered his world record in the event to 4:05.25 at the US trials in June.

"I saw the first few heats go out, and I kind of wanted to be the top seed for tomorrow, that is all I went out to do," Phelps said. "It's going to be a tough race between three or four of us."

Phelps easily won his heat before a near-capacity crowd at Beijing's Water Cube aquatic centre, where the crowd greeted his arrival on deck with enthusiasm but saved their loudest cheers for Chinese competitors.

The 23-year-old American, who is gunning for compatriot Mark Spitz's record of seven gold medals at one Games, was never challenged, with Italian Luca Marin second-fastest in his heat in 4:10.22.

Hungarian Laszlo Cseh, who set the European record at a meeting in France in June, was the second-fastest qualifier for Sunday morning's final, winning his heat in 4:09.26.

Marin was third-fastest overall and American Ryan Lochte, reckoned one of the chief challengers to Phelps in both the 400m and 200m medleys, was fourth-fastest overall, winning his heat in 4:10.33. Cseh said the times promised an explosive final.

"I think if someone wants to win this race they are going to have to swim under 4 minutes 05 seconds," he said. "It was a good time, but I am tired now," Cseh said. "I will try everything I can to beat those guys. It's going to be very, very difficult."

Lochte said it would take nothing less than "the perfect race" to deny Phelps a first gold of the Beijing games. "It's going to be a tough one for sure, but I will give him a run for his money," Lochte said.

Hungarian Gergo Kis, Italy's Alessio Boggiatto, Canadian Brian Johns and Brazilian Thiago Pereira rounded out the field for the final that could set Phelps on his path to glory.

Australian Jessicah Schipper was the top qualifier for the semi-finals in the women's 100m butterfly. In later heats, Australian Grant Hackett and South Korean Park Tae-Hwan jockeyed for position in the men's 400m freestyle while American Katie Hoff was trying to get her own multi-medal campaign underway in the women's 400m medley.