The seeding of the semi-finals brought the past two Olympic champions together in the first race.
The 2012 gold medallist, Kirani James from Grenada, got the bragging rights over his predecessor, USA’s LaShawn Merritt, although both men were certainly running somewhat within themselves.
James, as has so often been the case, ran a fast first 200m and had a clear advantage coming around the second bend.
Merritt, the 2008 Olympic champion, pushed hard for about 50 metres to let James know he was there before easing up, both men knowing that they were as good as certain of qualifying by right. Merritt let James exert himself a little more to take the win in 44.02, which was eventually the fastest time of the semi-finals.
Merritt took second in 44.21 as Botswana’s Karabo Sibanda had the race of his life and a magnificent final 100 metres to take third in a personal best of 44.47, more than half a second quicker than he had ever run before, his previous best being 45.15.
The second heat saw Trinidad and Tobago’s emerging star Machel Cedenio, running in lane five, push hard over the first 250 metres.
World champion Wayde van Niekerk was two lanes inside Cedenio and got on his shoulder coming off the second bend and briefly edged ahead but Cedenio surged again as the South African eased his foot off the pedal and let the 2014 world U20 champion take the win.
Cedenio clocked 44.39 with Van Niekerek 0.06 further back.
Two-time world indoor champion Pavel Maslak came flying down the home straight to pick up third but had left his effort too late and his time of 45.06 was not enough to ultimately see him through.
The third heat saw the Granadian number two Bralon Taplin do all the hard work from lane six and tow the rest of the field around the Olympic stadium.
Taplin was clearly tying up in the final 50 metres but did enough to hang on to win in 44.44, just 0.06 shy of his personal best, ahead of Great Britain’s Matthew Hudson-Smith in lane eight, who made a huge improvement to take second place in a personal best of 44.48.
The Briton's late surge over the final 50 metres saw him just edge out Bahrain’s Ali Khamis Khamis, who briefly held the lead coming off the final bend, by 0.01 but the latter went through as a non-automatic qualifier after finishing third in a national record of 44.49.
The impression from the semi-finals is that the medal battle in Sunday’s final should still be between the three men who climbed the podium in Beijing last summer – Van Niekerk, Merritt and James – but be prepared for a spate of personal bests for the men behind them.
Phil Minshull for the IAAF