Michael Phelps' old foe Milorad Cavic predicted the US superstar would produce "something special" Friday in the 100m butterfly final, the last individual race of his epic Olympic career.

Phelps lines up against Cavic and young challenger Chad le Clos vying to complete a second individual Olympic "three-peat" a day after he became the first man to win the same solo swimming event at three successive Games.

"He didn't have it on his goal sheet, but for me that was important," Phelps' coach Bob Bowman said of the unprecedented treble. "I always wanted him to do three just because that was cool."

"Phelps is out of our league," acknowledged Cavic, famously beaten by Phelps in a 100m fly photo-finish in Beijing by one one-hundredth of a second.

"It's not fair that I'm talking for everybody, but I'm expecting something special," added the Serbian.

Meanwhile South Africa's le Clos, who plunged past Phelps at the finish to end the American's dominion in the 200m fly, was just looking forward to another race against his idol.

"I'm next to him again, so that will be another dream for me to swim next to Michael Phelps," said le Clos, who downplayed his chances of pulling off another upset in the shorter race.

The 100m fly highlighted four finals on the penultimate day of swimming competition at the Aquatics Centre, all featuring defending Beijing gold medallists.

British hopes were pinned on Beijing winner Rebecca Adlington in the 800m freestyle and world record-holder Cesar Cielo was out for a golden repeat in the men's 50m free.

In the 200m backstroke, two-time defending champion Kirsty Coventry was a longshot in a final featuring 17-year-old US star Missy Franklin, who is in pursuit of a backstroke double after her triumph in the 100m race.

The US also sends a strong contender in 400m medley silver medallist Elizabeth Beisel, with Russian Anastasia Zueva and Australian Meagan Nay also in the medal mix.

Friday's last day of heats saw world record-holder Sun Yang of China lead the way into Saturday's 1,500m freestyle final ahead of Beijing gold medallist Oussama Mellouli of Tunisia and Commonwealth Games champion Ryan Cochrane of Canada.

Dutch speedster Ranomi Kromowidjojo stayed on track for a freestle double as she led the way into the semi-finals of the women's 50m free a day after winning 100m gold, and the US men made it safely into Saturday's final of the men's 4x100m medley relay -- where Phelps will have one last chance for gold.