Le Clos finished second behind Takeshi Matsuda of Japan in the faster of two semifinals in the men's 200 metres butterfly, clocking a national record of one minute, 54.34 seconds (1:54.34).
The 20-year-old South African, who was fifth in the men's 400m individual medley final on Saturday, qualified second fastest for Tuesday night's final.
American Michael Phelps won the other semifinal in 1:54.53.
"I'm obviously happy with the time. It is fantastic," Le Clos said.
"I was so relaxed going into this race. I don't know why, but I was actually nervous that I was too relaxed.
"I guess it worked out for me and I just hope I can repeat tonight's performance [on Tuesday]."
Earlier there were disappointing performances in the pool with Karin Prinsloo crashing out in her heat of the women's 200m freestyle.
Prinsloo finished seventh in 1:59.24 in her Olympic debut, but she will have another chance to make a splash in the 200m backstroke event.
Meanwhile, Kathryn Meaklim's Olympic campaign came to an end as she failed to progress beyond the heats of the women's 200m individual medley. She finished seventh in her heat in 2:15.25.
Elsewhere, a clinical hat-trick by Australian midfielder Jamie Dwyer helped his side to a 6-0 victory over the South African men's hockey team in their Pool A opener.
The South Africans conceded two penalty strokes and too many set-pieces as world number one Australia stamped their authority and set the benchmark for the rest of the tournament.
The South African women's rowing pair of Lee-Anne Persse and Naydene Smith missed out on qualifying for the A final, and a chance for a medal, after finishing fifth in the repechage in 7:18.96.
Persse and Smith will compete in the B final on Wednesday.
The country's only entrant in archery, Karen Hultzer, was eliminated in the first round of the women's individual event.
Hultzer, who was 46th of 64 competitors in last week's ranking round, won the third set against former European Championships bronze medallist Pia Lionetti, but the South African lost the match 6-2.
In beach volleyball, the men's team of Freedom Chiya and Grant Goldschmidt went down in straight sets, in their second successive group game, to the Latvian pairing of Aleksandrs Samoilovs and Ruslans Sorokins.
Judoka Gideon van Zyl was eliminated in the last 32 stage of the men's 73kg discipline by Rustam Orujov of Azerbaijan in four minutes and 34 seconds, bringing an end to his Olympic campaign.
The badminton team of Michelle Edwards and Annari Viljoen were eliminated from the women's doubles competition after going down to Greysia Polii and Meiliana Jauhari of Indonesia – their third successive defeat in the group stages.
And equestrian Alex Peternell was lying 56th of 59 remaining competitors after two events – dressage and cross country – in individual eventing.
Peternell, who won a case at the Court of Arbitration for Sport to be included as the only horse rider in the South African team, will compete in the final eventing discipline (jumping) on Tuesday.
China stretched their lead at the top of the Olympic medal table on Monday as American swimming sensation Missy Franklin lived up to the hype with her first gold of the Games.
After a stellar opening weekend which saw them bag six titles, China scored wins in artistic gymnastics, diving and women's weightlifting to take their total to nine on a day marred by officiating controversies.
China's men's gymnasts endured a disastrous qualifying round but swept back to form to win the all-round team event and retain the gold won in Beijing four years ago.
But the biggest cheers were reserved for Britain's men, who finished with a bronze – the hosts' first medal of any colour in the team gymnastics event since Stockholm 1912.
Britain had initially finished in the silver medal position before being relegated into bronze as Japan moved up to second after an appeal, with Ukraine dropping out of the medals altogether.
Elsewhere, China's Li Xueying set two new Olympic records as she crushed her rivals on the way to clinching gold in the women's weightlifting 58kg class.
At the Aquatics Centre, China's Cao Yuan and Zhang Yanquan took advantage of a blunder by British duo Tom Daley and Pete Waterfield to win the 10m platform synchronised diving gold.
The British pair had been on course for the host nations' first gold until a messy dive from Waterfield let the Chinese teenagers back into the contest.
Daley later refused to blame Waterfield for his mistake that likely cost Britain a medal.
"We're a team, that's it, full stop," said the 18-year-old. "You win as a team and you lose as a team."
The Olympics lived up to its tradition of providing abundant drama in sports which rarely enjoy a global spotlight, with amazing scenes in the fencing competition at the Excel Centre.
South Korean fencer Shin A-Lam staged a dramatic, lonely sit-down protest for more than one hour after losing her epee semifinal against Germany's Britta Heidemann, convinced she had been the victim of an officiating blunder.
Shin sat sobbing disconsolately on the piste as her coach protested her defeat to no avail. She eventually returned to lose her bronze medal match.
"I am very emotional. I should have won," said Shin, the 11th seed.
Away from the medals, Great Britain trounced Argentina 4-1 in the men's field hockey, a game which followed recent tensions between the two nations on the 30th anniversary of the Falklands War.
Switzerland footballer Michel Morganella became the second competitor to be sent home for posting racist abuse on Twitter after insulting South Korea's players on the micro-blogging site.
"Michel Morganella has discriminated against, insulted and violated the dignity of the South Korean football team, as well as the South Korean people," said Switzerland Olympic team chef de mission Gian Gilli.
Greek triple jumper Voula Papachristou was kicked out of the Olympics last week for a comment which poked fun at Africans living in the country.
Meanwhile under-fire London Olympic organisers (LOCOG) continued to face criticism over the banks of empty seats which have been seen across various venues since the Games got under way.
Some 3 000 tickets from international sports federations were "put back in the pot" and sold to the public Sunday, LOCOG said amid growing public anger over empty seats. – Sapa-AFP