Michael Phelps safely cleared the first hurdle in his quest to win eight gold medals on Saturday when he broke his Olympic record to qualify fastest for the 400 metres individual medley final.
The American cruised through his heat in four minutes 07.82 seconds, wiping 0.44 seconds off the Olympic record he set to win the gold medal in Athens four years ago.
"I wasn't expecting heats to be that fast, after seeing the first two heats I wanted to be top seed going into tomorrow," said Phelps, who added that he had no idea what time it would take to win.
"It's going to be a race between three or four of us. I'm looking forward to it."
Hungarian Laszlo Cseh, who finished third behind Phelps at Athens then won the world title in Montreal a year later when Phelps skipped the event, was second fastest in 4:09.26.
Italian Luca Marin was third in 4:10.22 while American Ryan Lochte, who looms as the one of the biggest dangers to Phelps' bid to break compatriot Mark Spitz's 1972 record and win an unprecedented eight golds, was fourth in 4:10.33.
Phelps' time was the third fastest and just 2.57 outside the world record of 4:05.25 he set at the US trials six weeks ago to book his place in Sunday's final.
Clean shaven but not wearing a full bodysuit, he was under world record pace halfway through the backstroke leg but eased down over the final freestyle leg to conserve his energy for the final.
Lochte was happy with his performance.
"It was alright...I got it out of the way. It was a good race, my best prelim swim, I'm happy with that," he said.
"It (the final) is going to be tough, but I'm going to give Michael a run for his money."
Cseh also looked forward to the showdown.
"I think if someone wants to win they have to go under 4:05," he said. "I'll try to beat Michael. I will try to go with no pressure, I'll give it everything."