Le Clos, who pipped Phelps to win the 200m butterfly on Tuesday night, was the second-fastest in 51.42sec, winning his semifinal at the death after turning in fifth spot. Phelps won the other semifinal in 50.86s, but Le Clos can expect stiff competition for podium positions from Tyler McGill of the US and Serbian Milorad Cavic, who was beaten by Phelps in 2008 by one-hundredth of a second.

''I think tomorrow night will be a little unrealistic to try and win. But I can try give it a good race. I'm next to him again so it's another opportunity to swim next to Michael Phelps. I'm just happy about that.''

Le Clos, wanting to focus on the two-lap fly race, scratched from last night's 200m individual medley final, where Phelps beat arch-rival Ryan Lochte.

Veteran Roland Schoeman will also vie for silverware in the 50m freestyle tonight.

For the first 100m Suzaan van Biljon threatened to win a second medal, lying third at the halfway mark of the women's 200m breaststroke. But it wasn't to be as she faded badly and finished seventh. At least Van Biljon can hold her head high, being the country's only female swimmer to have made a Games final since 2000 and in the process uncovering fresh inspiration to keep pushing for Rio 2016.

The race was dominated by reigning champion Rebecca Soni, who burned up the pool in a 2min 19.59sec world record, and that was a pace the South African couldn't keep up with.

Van Biljon was fourth at 150m and she touched in seventh place 2:23.72, a fraction slower than the African record she cracked in the semifinals the previous night.

''I had a really comfortable first 100, but I did die the last 50m,'' said Van Biljon. ''I've got a lot of work to do in the second hundred, which makes me really excited.''

Track and field begins today with Khotso Mokoena, SA's only medallist in Beijing, looking to qualify tonight ahead of tomorrow's long jump final. LJ van Zyl and Cornel Fredericks are in action in the heats of the men's 400m hurdles.