By Nick Mulvenney

RIO DE JANEIRO (Reuters) - Allyson Felix showed no discomfort from her ankle as she eased through the first round of the 400 meters heats on Saturday, taking another step towards what would be a fifth Olympic gold for the American.

Hampered by the injury she sustained this year in a freak gym incident, the 30-year-old Californian missed out on defending her Olympic 200 meters title when she failed to get in to the top three at the cut-throat U.S. trials.

That has made her bid to add the Olympic title over one lap to the world crown she already owns even more important and she started her campaign with a comfortable win in heat two, stopping the clock at 51.24 seconds after easing up over the last 50 meters.

"It feels good, I was focusing on first half," she told reporters.

"I've made good progress (with the ankle) since trials. It's fairly close to what it should be. Recovery is the key now.

"It's good to get out there on the track and get started. This race is completely different to the 200m. I just have to run hard and focus on doing the 400m now. I'll do my best."

Felix's time was only the sixth fastest of the morning behind a host of pretenders to the Olympic crown Sanya Richards-Ross will be unable to defend after suffering a career-ending injury at the U.S. trials.

American Phyllis Francis topped the time sheets in 50.58 seconds followed Bahrainis Oluwakemi Adekoya (50.72) and 18-year-old Salwa Eid Naser (51.06), a national record and personal best respectively.

Bahamian Shaunae Miller, who owns the best time of the year (49.55), also won her heat in 51.16 with the best European, Italian Libania Grenot, sixth fastest on the clock in 51.17.

"The race went good, I'm just fed up with this walk afterwards," Miller said of the complicated route the athletes have to negotiate through the media zone.

"Up so many stairs and then down so many. It's like a whole different workout."

All three Americans won their heats, Natasha Hastings came home in 51.31 to win the fifth race, as did Jamaicans Stephenie Ann McPherson (51.36), Christine Day (51.54) and Shericka Jackson (51.73).

The semi-finals take place on Sunday with the final on Monday.

(Editing by Ed Osmond)