Athens - South Africa's Roland Schoeman and Lyndon Ferns have qualified for the Olympic 50m freestyle semi-finals at the Athens Games on Thursday.
However, Pieter van den Hoogenband and Alexander Popov caused the biggest upset when they failed to advance from the heats.
Schoeman, who won the Olympic silver medal in the 100m freestyle on Wednesday, qualified for Thursday evening's semi-finals with a time of 22.41, while Ferns swan his heat in 22.53.
Newly-minted Olympic 100m freestyle champion Van den Hoogenband and freestyle sprint great Alexander Popov failed to advance from the heats at the Athens Games on Thursday.
For Popov, the 50m was a last chance at individual Olympic glory for the man who won back-to-back 100m free and 50m free doubles at the 1992 and 1996 Olympics.
For Van den Hoogenband, the 50m free was a chance to add to the gold he won with an emotional defence of his 100m title from Sydney on Wednesday night.
Van den Hoogenband's time of 22.56 was 17th, just missing out on the semi-finals.
Popov's time of 22.58 put him in a three-way tie for 18th.
"I'm not disappointed. I knew it could happen," said Van den Hoogenband, 26. "Yesterday (Wednesday) I was so, so happy.
"I was so relieved. It was such a big night for me yesterday to win the gold again."
Come Thursday Van den Hoogenband said, "I was so not-focused.
"Everyone was congratulating me, I went in the dining room and people were standing up cheering.
"I feel today was not real, It was a dream - sign this, sign that. And I was saying 'I have to race, focus, focus, focus.
"The 100 free final - that was the day."
But Popov strode off the pool deck looking grim.
It was the second Athens disappointment for Popov, 32, who failed to make it out of the semi-finals in the 100m free.
"It's not very good," Van den Hoogenband said of Popov's failure to advance. "He's a great, great swimmer, and he deserves a spot in the final."
The Russian sprint legend saw his eight-year domination come to an end in Sydney four years ago when he lost both his 100m and 50m titles.
The 32-year-old Russian had to settle for silver in the 100m free and saw his world record broken twice, but he bounced back from the loss of his Olympic titles by reclaiming both at the world championships in Barcelona last year.
That left him set for a bid to become the first man to win three Olympic titles in the same event - a feat accomplished by three women swimmers.
American Gary Hall, who shared the 50m gold with now-retired team-mate Anthony Ervin in Sydney, led the 50m heats with a time of 22.04sec.
