Mo Farah was surprisingly beaten by club runner Ellis Cross at the Vitality London 10,000 as he returned to competition after an 11-month absence.
The four-time Olympic champion, 39, was unable to out-kick Cross, who won by four seconds in 28 minutes 40 seconds.
Before the race, Farah said it would not “make any sense” to keep racing if he could not compete at the top level.
“I have beaten somebody who is like an idol,” Cross, who represents Aldershot, Farnham & District, told BBC Sport. Farah, who last raced in June 2021, was unable to secure a place at the Tokyo Olympics after fracturing his foot in the build-up to qualifying.
“Today was tough and Ellis did well to beat me,” Farah, 39, told BBC Sport.
“It is really important to test your body and see where you are. It has been so long since my last race.
“I love the sport and what I do. I have had a long career. Your body has to be ready. You have to be in the right frame of mind and compete with the guys. You have to say where you are… I ain’t young any more am I?”
Cross, from Polesworth in Warwickshire, said he “didn’t quite follow the script” in upsetting Farah in front of a London crowd that cheered him on in the 16,000-strong event.
As a club runner, the 25-year-old had to pay a £37 entry fee to run in the event.
“I can’t believe it,” he said. “I didn’t believe it until 20 metres from the finish. I thought he would just out-kick me.
“I can’t explain the feeling, it’s overwhelming.
“He is known around the world and I have beaten him over his prime distance. Nobody knows who I am but the crowds are cheering.”
Great Britain’s Eilish McColgan dominated the women’s field and finished just two seconds behind Paula Radcliffe’s 19-year-old British and European 10,000m record.
McColgan, 31, clocked 30:23, which was enough to at least set a Scottish record, bettering the mark of 30:39 set by her mum Liz in 1989.