London - South African swimmer Gideon Louw has booked his place into the men's 100m freestyle semi-finals at the 2012 London Olympic Games in a time of 48.29 while his fellow compatriot Graeme Moore missed the cut in his heat on Tuesday.

Nathan Adrian of the US qualified fastest in the 100m freestyle preliminaries at the London Olympics, topping a tightly bunched field that included three ties.

Adrian won his heat in 48.19 seconds while swimming in a lane next to world-record holder Cesar Cielo of Brazil. Cielo, the 2008 bronze medalist, tied for 10th with Konrad Czerniak of Poland in 48.67.

Cielo's time was well off his world mark of 46.91 set at the 2009 world championships during the peak of the high-tech body suit craze. Those suits have since been banned.

"It felt a little heavy, actually," he said. "But hopefully tonight I'm going to bring some heat and I'll swim better to make the final."

Louw of South Africa was second-quickest in 48.29, followed by Sebastiaan Verschuren of the Netherlands at 48.37.

World champion James "The Missile" Magnussen of Australia advanced in fourth at 48.38. No other swimmer in a textile suit has gone faster than the 47.10 he posted at his country's trials in March.

But Magnussen's opening leg of the 4x100 freestyle relay on Sunday raised some doubt about whether he can add the Olympic title to his world championship. The Aussies were favored but slipped to fourth.

"I took a fair hit in the relay. I am still trying to bounce back from it," he said. "It hurt my pride as much as anything else. A lot of my competitors have never seen me lose."

Adrian swam the leadoff leg for the Americans in 47.89, going out faster than Magnussen to give the US an early lead. The US settled for silver after Ryan Lochte was caught on the final lap by Yannick Agnel of France.

"It wasn't that I was nervous. I just got too worked up over it," Magnussen said. "I just need to stay relaxed and be confident. They probably think they can beat me, but I am going to fight for this one."

Cielo wasn't ready to write off the Aussie.

"That was one mistake. You can't put those guys out of contention," he said. "I think James is still one of the favorites. I would put him and Agnel down as hands-down the favorites."

Agnel tied James Roberts of Australia for 12th in 48.93. American Cullen Jones advanced to the 16-man evening semi-finals in ninth at 48.61.

The other tie was for sixth between Brett Fraser of the Cayman Islands and Pieter Timmers of Belgium at 48.54.