American Allyson Felix bids for her 10th World Championship gold medal in the women's 400 meters on Wednesday.
The 31-year-old comes up against Shaunae Miller-Uibo of the Bahamas who dramatically pipped her on the line to the Olympic title in Rio last year.
Felix will equal the record of 14 World Championship medals shared by fellow sprinters Usain Bolt and Merlene Ottey if she finishes in the top three.
American Kerron Clement is attempting to win his third world title in the 400 meters hurdles after qualifying fastest for the final ahead of young Norwegian Karsten Warholm.
The women's shot put is the third gold medal up for grabs on Wednesday and South African Wayde van Niekerk continues his bid for the 400/200m double when he runs in the semi-finals of the shorter event.