Rio track and field preview: Five track stars to watch

Bolt heads into the meet as a six-time Olympic gold medalist, with his sights set on another three. He is the two-time defending champion in the 100- and 200-meter sprints, and he could become the first Olympian to win three consecutive 100-meter golds.

The world's fastest man swept his three events (100, 200 and 4x100 relay) at the 2013 and 2015 world championships. He has lost only one 100 race since the 2012 Olympics.

But the biggest question surrounding Bolt is his health. He withdrew from Jamaica's Olympic trials in July, citing a hamstring injury. But he has often entered meets citing injuries, and it hasn't stopped him.

Eaton established himself as the world's greatest athlete in 2012, when he broke the decathlon world record and won the Olympic gold medal in London.

Since then, he has only gotten better. He collected his first two decathlon world championships and, in 2015, broke his own decathlon world record.

In Rio, Eaton will attempt to become the first decathlete to successfully defend an Olympic gold medal since Great Britain's Daley Thompson in 1984.

Felix will run the 400 meters at the Olympics for the first time. She will be the favorite in the event after clocking 49.68 seconds, a world-leading time, at the trials. She missed a spot in the 200 by 0.01 seconds in the trials.

Gatlin, above, won the 2004 Olympic 100, but he has yet to defeat Bolt in an Olympic final. Rio will most likely be his final opportunity.

Gatlin was third to Bolt and Jamaica's Yohan Blake in 2012, after serving a four-year ban for substance abuse. At the 2015 world championships, he finished second to Bolt in both the 100 and 200.

At 34, Gatlin became the oldest man to make a U.S. Olympic team in a sprint event since 1912, and a victory would make him the oldest person to win the 100 and the first to win the event 12 years apart.

The Jamaican sprinter, who is nicknamed the "Pocket Rocket," was the Olympic 100 champion in 2008 and 2012. In Rio, she will try to become the first woman to win three consecutive Olympic 100 gold medals.

But she will not run the 200, an event in which she earned an Olympic silver medal in 2012. She scratched from the event at the Jamaican trials, reportedly because of a toe injury.