Usain Bolt and controversial Justin Gatlin will get track and field the world championships off to an explosive start when they take their rivalry onto the Beijing track in the 100m heats on Saturday. Bolt claimed a golden treble at the Beijing Olympics in the Bird's Nest stadium, winning nine of 10 individual world and Olympic sprint titles since 2008. Gatlin is in form, but he cuts a contentious figure after serving two doping bans.