Kenyan 800 metres star David Rudisha was upstaged by Ethiopian tyro Mohammed Aman at the Zurich Diamond League meeting, the season finale.
Rudisha was bidding to better his own world record of a minute 40.91 seconds he set when winning the gold at the London Games earlier this month.
But no one had counted on the kick of 18-year-old Aman, who finished sixth at the Olympics but who importantly last year became the first and last man to defeat Rudisha since 2009.
Aman held on for a convincing win in a personal best of 1:42.53, with Rudisha timing 1:42.81.
