South Africa's Wayde van Niekerk flaunted his superb form on Tuesday night, taking the first step in his attempt at a historic double at the IAAF World Championships, London 2017.
He completed the race in 43.98, covering the distance 0.43 ahead of Bahamas athlete Steven Gardiner.
"I hope the fans enjoyed that," Van Niekerk said. "I'm used to the lactic (acid in my legs) but I need to recover from that effort and I'll be fine.
"I've got a good team who will help me recover well for the 200m and I'm looking forward to that."
Elsewhere on the tracks...
Former World Championships finalist Wenda Nel was third in her semifinal of the 400m Hurdles discipline in 55.70, missing out on a place in the medal battle.
Another speedster, Justine Palframan, finished fourth in her first-round 200m heat in 23.35, progressing to the semifinals.
"Congratulations to Wayde Van Niekerk for retaining the title he won at the 2015 IAAF World Championships in Beijing," said Athletics South Africa President, Aleck Skhosana.
"He put behind his achievement tonight and focussed on the forthcoming 200m semifinals and final in order to achieve his ambition of doubling in the 400m and 200m.
"We thank the coach and the entire team that supported him throughout the rounds and preparing him to stay focussed in order to win a gold medal for SA."
After five days of competition, Team SA team had secured four medals (two gold, one silver and one bronze) and moved to third position on the medals table.