By Nick Mulvenney

RIO DE JANEIRO (Reuters) - Kirani James, Wayde van Niekerk and LaShawn Merritt moved a step closer to their highly-anticipated 400 meters showdown when they all eased through the first round of heats at the Rio Olympic Stadium on Friday.

Reigning Olympic champion James ran the fastest lap of the night to win the sixth heat in 44.93 seconds but refused to accept that he was one of the favorites.

"I feel good, I'm just happy I qualified for the semi-finals," said the Grenadian after clocking one of only two sub-45 second times in the seven races.

"I have not won a world title since 2012 so I am hardly the favorite, I am just a competitor."

South African Van Niekerk, who beat Merritt and James to win the world title in a thrilling final in Beijing last year, won his heat in 45.26 ahead of London Olympic silver medalist Luguelin Santos of the Dominican Republic.

Former Olympic champion Merritt of the United States, running in lane eight, clocked 45.28, easing up to win the heat and take his place in the semi-finals.

Merritt, who will also run the 200 meters in Rio, was unable to defend the Olympic title he won in Beijing at the last Games in London after suffering a hamstring injury in the heats.

"I wasn't able to compete in 2012 so I'm here to win it," he said.

"I expect it to be a fast final and I expect to be at the top of the podium. It's a round at a time, before I even think about the 200."

Machel Cedenio of Trinidad and Tobago ran the second fastest time of the evening (44.98) with the Bahraini winner of the final heat, Ali Khamis Khamis, third quickest in 45.12.

James Chiengjiek, a South Sudanese refugee competing under the Olympic flag, finished last in the fourth heat in 52.89 seconds, the slowest of time of the day.

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

(Editing by Ed Osmond)