RIO DE JANEIRO (AFP) - Usain Bolt will race the 200m final on Thursday after comfortably winning his semi-final, but there was no place for American rival Justin Gatlin as he failed to qualify.

Gatlin, who has served two doping bans, tied up late in his semi-final, only finishing third in 20.13 seconds and missing out on one of the eight final berths.

It was a different story for Bolt, however, who clocked a season's best of 19.78sec in the next step on his quest for an unprecedented sprint "triple triple".

The towering Jamaican eased up a full 40m from the line, to advance along with Canadian Andre De Grasse, setting a national record of 19.80sec as the second automatic qualifier.

Bolt won a third successive 100m title on Sunday to set up his bid to repeat his haul of three gold medals in the 100m, 200m and 4x100m relay at both the Beijing and London Olympics in 2008 and 2012 respectively.

Gatlin won silver behind Bolt in the 100m, but looked uncomfortable throughout his race.

The US challenge will now fall to Lashawn Merritt, the 400m specialist who clocked 19.94sec to win his heat ahead of France's Christophe Lemaitre.

Also qualifying for the final are Panama's Alonso Edward, Briton Adam Gemeili, Azerbai-born Turk Ramil Guliyev and European champion Churandy Martina of the Netherlands.