Amusan started her quest for an Olympic medal on Wednesday when she won Heat 1 of the 100m women's hurdles event
Nigeria's world record holder in the women's 100m hurdles, Tobiloba Amusan, won her heat with a time of 12.49s, ahead of the USA's Alaysha Johnson.
Amusan holds the world record in the event, which she set in Oregon at 12.12s.
The 27-year-old is Team Nigeria's best hope of winning gold in athletics.
In Heat 2, defending Olympic champion Jasmine Camacho-Quinn won in 12.42s, followed by Cindy Sember and Pia Skrzyszowska, respectively.
In Heat 3, the USA's Masai Russell came in first with a time of 12.53s, followed by Nadine Visser of the Netherlands in second and Cyrena Samba-Mayela of France in third place.
Eighteen athletes are through to the semi-final, scheduled for Friday.