Haile Gebrselassie suffered a shock defeat in the 10,000 metres final.
The Ethiopian distance runner - bidding to win his fifth successive world title - finished third in a race won by Kenya's Charles Kamathi.
Gebrselassie's Ethiopian team-mate Assefa Mezgebu took the silver medal.
The defeat ended Gebrselassie's unbeaten run which had stretched back for eight years.
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It was his first outing since taking the Olympic 10,000 metres gold medal in Sydney.
Gebrselassie - who may now run in next year's London Marathon - took time off after the Olympics and in November had an operation to clear out a damaged Achilles.
But his long lay-off proved too much to bounce back from, and Kamathi took gold with a winning time of 27 minutes and 53.25 seconds.