SHANGHAI -- Britta Steffen of Germany dropped out of the world championships after swimming the slowest qualifying time in the 100-meter freestyle on Thursday.
The defending champion's time of 54.86 seconds was only good enough for the 16th and last spot in the evening semifinals. It was Steffen's second poor swim in Shanghai, having led off Germany's 400 free relay that earned a bronze last weekend.
"I'm very disappointed, but I have no explanation," she said. "It's not what I expect from myself, but I have to react to this the same way I reacted to the successes."
Steffen said she felt fine in training leading up to the meet and once she arrived in Shanghai, but wasn't able to bring any speed to her races.
Her decision means she won't be part of Germany's team in the 4x100 medley relay on Saturday.
"If she doesn't want to, there's nothing we can do," said Lutz Buschkow, managing director of the German swimming federation. "I'm sure that her teammates will understand her decision."
She took nearly 1½ years off after the 2009 worlds in Rome -- where she won the 50 and 100 free events -- and didn't return to long-course racing until the German trials in June.
"We have to protect her, that's why she won't swim again," Steffen's coach Norbert Warnatzsch said. "We know definitely that she won't swim faster."
Steffen's withdrawal wasn't the only bad news for Germany. Its women's 4x200 free relay finished 10th in Thursday's prelims and failed to make the final.
Femke Heemskerk of the Netherlands had the fastest 100 free time of 53.75. Jeanette Ottesen of Denmark was second at 53.88. Heemskerk's teammate, Ranomi Kromowidjojo, was third in 54.10.
American Dana Vollmer was fourth, and teammate Natalie Coughlin 12th.
"I'm definitely keeping my eyes on (Kromowidjojo) and Natalie and Femke," Vollmer said.
American Tyler Clary easily advanced in the preliminaries of the 200 backstroke, with teammate Ryan Lochte posting the fourth-fastest time.
Clary led the way in 1 minute, 56.32 seconds, putting the United States in good position to defend the title its won eight times in the event, including the past six consecutive world meets.
Clary missed out on the 200 fly final, won by Michael Phelps on Wednesday.
"It was upsetting to see that I got second in my semifinal heat and still didn't make it back because seven guys went faster in the second heat," he said. "Unfortunately, I got caught with my pants down. Definitely not going to make that mistake tonight."
Lochte, the bronze medalist two years ago behind now-retired champion Aaron Peirsol, touched in 1:57.34. Lochte won the title in 2007.
Peter Bernek of Hungary was second at 1:57.23, followed by Stanislav Donets of Russia. Japan's Ryosuke Irie, the silver medalist in 2009, was sixth.
The United States also had the top qualifier in the 200 breaststroke, with Olympic champion Rebecca Soni clocking 2:23.30 and setting herself up for a sweep of the breaststroke events.
No one has seriously challenged Soni in her races, so she tries to keep a blank mind while churning toward the wall.
"I'll count strokes even though I don't really care what the number is I'll count just to keep my mind moving," she said. "For me, the most important thing is to not think and relax and not let any pressure get to me."
Rikke Pedersen of Denmark was well back in second at 2:25.86. American Amanda Beard, the 2003 champion, was fourth.
Beard swam next to Soni in their heat.
"I know how she swims, I know she's going to be strong, and I just figure if I can stay somewhat close to her, then I'm doing all right," she said.
The United States coasted through the 4x200 free relay prelims. Missy Franklin, Katie Hoff, Jasmine Tosky and Dagny Knutson finished in 7:50.46.
Knutson was on the team that lost to China two years ago.
"It's always in the back of our minds that we're on a mission to get a gold again," she said. "We've had such a great history in this relay and we want to maintain that."
Canada's team of Julia Wilkinson, Brittany Maclean, Samantha Cheverton and Barbara Jardin was second-quickest at 7:52.05, followed by Hungary at 7:52.12. China was fourth.
In the men's 200 breast, Giedrius Titenis of Lithuania was fastest at 2:10.33.
American Eric Shanteau, the silver medalist two years ago, was third at 2:10.77 after waiting five days to swim.
"Once we got here, people started preparing for the meet and I just had to sit idly by and not get too excited," he said. "I'm just glad to finally get a race under my belt. It definitely didn't take all my energy to do that."
Defending champion Daniel Gyurta of Hungary was fourth. Two-time champion Kosuke Kitajima of Japan was sixth. American Elliott Keefer finished 18th, two spots out of the semifinals.
Norway's Alexander Dale Oen, the 100 breast champion, didn't start in the 200 because he was sick.
Phelps returns to the pool in the evening for the 200 individual medley final, a showdown with Lochte.