Michael Phelps failed to make it past the 100m freestyle heats and was then disqualified in the 100m backstroke at the Stockholm short-course World Cup.
Competing for the first time since August's World Championships, Phelps - a winner of eight Olympic golds in 2008 - was 16th-fastest on Tuesday.
But, unlike others, the American had chosen not to wear a hi-tech bodysuit, banned by Fina from 1 January 2010.
Joe Roebuck and Lizzie Simmonds claimed gold for Britain on the opening day.
Roebuck, 24, was almost a second ahead of his 400m individual medley competitors to touch home in four minutes 3.29 seconds, with Chad Le Clos of South Africa taking silver.
Eighteen-year-old Simmonds, based in Loughborough alongside Roebuck, set a new European record in her 200m backstroke, finishing in a time of 2:02.01 ahead of Australian Belinda Hocking.
Stockport's Michael Rock lowered the British record twice on his way to winning bronze in the 200m butterfly.
606: DEBATEHis time of 1:51.32 placed him behind Brazil's Kaio Alemeida, who won gold in a world record 1:49.11, and Nikolay Skvortsov of Russia.
Phelps had earlier been 1.84 seconds behind the best 100m freestyle time, set by Sweden's Stefan Nystrand.
The hero of the Beijing Games could only record a time of 47.77 seconds. Meanwhile, Frenchman Frederic Bousquet (47.53 secs), Australia's Eamon Sullivan (47.38) and Filipo Magnini of Italy (47.74) also failed to reach the final.
Until 1 January, swimmers can choose either to stick with the bodysuits, which were worn when there was an avalanche of world records in Rome, or switch back to more conventional attire.
The ongoing meeting is Phelps' first in a short-course event since 2006 and his first event in Europe since 2001, also in Stockholm.
He is entered for five races in Sweden and will go on to compete in Berlin at the weekend for the next leg of the World Cup - before joining the US team to take on Europe in the "Duel In The Pool" event in Manchester in December.
"I know I'm not in the best shape and I should be in better shape," said Phelps, whose backstroke disqualification was delivered for having spent too long swimming under water.
"I didn't hit my first wall right, so it's kind of disappointing but I'm happy overall."
Phelps did later qualify for the 100m medley final but came third in a time of 52.14 seconds, behind winner Gerhard Zandberg of South Africa.
The 24-year-old Phelps was sporting a goatee beard - a further suggestion he is using Stockholm as a stepping-stone to bigger events down the line.
Meanwhile, Jing Zhao of China set a world record in the women's 50m backstroke in 26.08. She beat the previous world record of 26.17, set by Marieke Guehrer of Australia four days previously in Moscow.
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