TOKYO - Former Olympic champion Kirani James of Grenada sounded a warning to his fellow medal hopefuls in the men's 400m by posting the quickest time in the semi-finals on Monday.

Defending champion and world-record holder Wayde van Niekerk of South Africa crashed out, however, after finishing fifth in the third semi-final at the Olympic Stadium.

Van Niekerk suffered a number of injury setbacks since his world record-breaking run five years ago in Rio de Janeiro and his exit was not surprising given he made it to the semi-finals after finishing a lowly third during his heat.

World champion Steven Gardiner of the Bahamas overtook Van Niekerk and his other opponents with smooth strides in the last 50 meters to win the race and qualify third-fastest in 44.14.

"I felt very comfortable. I have more to go for the final. Get in there," Gardiner told reporters. "My confidence is so high right now. It was a good semi and a good heat. Now we have one more round and then we all get to go home."

James, who claimed the 400m gold at the 2012 London Games and a silver behind Van Niekerk in Rio, won the evening's first semi-final in a season-best time of 43.88 seconds.

"It would be perfect to get a third (medal)," he said. "It's going to be competitive. I'm going to go out there and make my country proud. My body is good, can be better."

Colombian Anthony Zambrano, silver medalist at the 2019 World Championships in Doha, pushed James all the way and set a new South American record of 43.93 to qualify second fastest.

The United States won seven straight golds in the event from 1984 to 2008 but the last two have eluded them, going to James and van Niekerk.

The Americans will have two challengers in Thursday's final in Michael Cherry and Michael Norman as they look to re-establish their dominance.

Cherry, who had posted the quickest time to advance to the semi-finals, won his race to advance with a time of 44.44 ahead of Jamaican Christopher Taylor.

Norman, who has the second-fastest time this year of 44.07, was second behind Gardiner.

Isaac Makwala of Botswana and Dutchman Liemarvin Bonevacia were the other two men to qualify for the final.

Belgium's Kevin Borlee withdrew from the semi-finals with a hamstring injury.