Usain Bolt ran his fastest time of the season to win his Olympic 200m semi-final in 19.78 seconds as Justin Gatlin missed out on a place in the final.
Jamaica’s Bolt has already won the 100m and is now aiming for an eighth Olympic gold by retaining his 200m title.
But American Gatlin, who won silver behind Bolt in the 100m final, only finished third in his semi-final.
However, there was a shock elsewhere in the semi-finals as Gatlin failed to make it through with a hugely disappointing run.
Gatlin had looked well placed to reach the final as he led the third heat coming off the bend, but he eased off prematurely and was overhauled in a mass finish on the line and his 20.13 was not good enough to make it as a fast loser.
“Everybody is in shock, I don’t know,” said Bolt after watching his sprint rival crash out.
Jamaican Yohan Blake, a former 100m world champion, also failed to reach the final, finishing sixth in the same semi as Gatlin.
Fellow American LaShawn Merritt was the only other man under 20 seconds and he also looked strong and comfortable going into Thursday’s final.