Nick Willis came a disappointing ninth in the men's 1500 metres in London on Tuesday and has apologised to New Zealand for not doing better.
The 29-year-old won silver at the Beijing Games in 2008 started well, but failed to keep pace when the final push began. The New Zealand captain says the conditions were fine for the race and he was disappointed in his performance.
Willis finished in a time of 3min.36s - six seconds slower than he ran in Monaco in July this year.
However, it was slowly run race, with Algeria's Taoufik Makhloufi kicking clear on the last lap to win easily in a time of 3min 34s.
Makhloufi had been thrown out of the event for not trying in his 800m heat, but was then reinstated when a doctor said he had actually dropped out because of injury.
American Leonel Manzano finished strongly to take silver ahead of Moroccan Abdalaati Iguider.
Defending champion Asbel Kiprop, of Kenya, hoping to match London 2012 Games head Sebastian Coe's unique achievement of winning the blue riband event of the track twice, trailed in last place.
Running commentator Roger Robinson says Nick Willis's loss is hard to fathom.
"It's really an extraordinary result, an extraordinary race. When it came to it, with 300 to go his legs just weren't at home and it's really unfortunate, I'm really sorry.
"We all feel gutted and it's a very hard race to explain."
Willis finished third at the Beijing Games, but was promoted to second after the winner, Bahrain's Rashid Ramzi, tested positive for a blood boosting drug and was disqualified.
He won gold for the event at the 2006 Commonwealth Games and bronze at the 2010 Commonwealth Games.
The Wellingtonian had planned to compete in both the 1500m and 5000m, but decided to concentrate on the shorter race - his preferred event.