Turkey's Asli Cakir won Olympic 1500m gold in a slow, messy race as Britain's Lisa Dobriskey finished down in 10th.
Cakir, banned for two years in 2006 for doping, clocked four minutes 10.23 seconds, ahead of compatriot Gamze Bulut and Bahrain's Maryam Yusuf Jamal.
USA's Morgan Uceny tripped and fell in a second consecutive global final.
After a pedestrian 75-second first lap, the pace picked up with 600m to go, stretching the field and cutting adrift Britons Dobriskey and Laura Weightman.
Cakir, 26, went to the front and held on down the home straight as Jamal tied up.
Bulut, 20, has enjoyed an extraordinary improvement over the past year, dropping her 1500m personal best from 4:18 to 4:01.
Her rapid advance culminated in a silver medal that few could have predicted last summer.
With Russia's Tatyana Tomashova, who returned from a two-year doping ban last summer, claiming fourth, it was a race that drew few plaudits from the 80,000 aficionados in the capacity Olympic Stadium crowd.
Dobriskey, world 1500m silver medallist in 2009, has once again struggled with injury this year and found the injection of pace too much.
But Weightman, at just 21, will take great pleasure at making the Olympic final in her breakthrough year.