With just the top two advancing automatically per heat, plus the next eight fastest across the eight heats, there was little margin for error but all the main contenders could say: “mission accomplished”.
On a sunny morning with negligible wind, ideal conditions for 400m running, Jamaica’s 2014 Commonwealth champion Stephenie-Ann McPherson ran a controlled race to secure the heat one win in 51.36 from Nigeria’s Patience Okon George, who was second in 51.83.
Four-time Olympic champion Allyson Felix began her quest for a maiden Olympic 400m title with a straight forward heat win in 51.24.
Running in long blue socks, the US sprint legend switched off the after-burners down the home straight only to gently push on the accelerator over the final 30 metres to ensure the heat win by 0.16 from Olha Zemlyak, the former European 400m silver medallist, of Ukraine.
Amid a rousing chant of “Brasil, Brasil” from the passionate home crowd for Jailma De Lima, the third heat got underway with a huge roar from inside the Olympic Stadium.
Sadly, for the home fans, De Lima made little impact winding up sixth in 52.65, but out in front USA’s Phyllis Francis produced a storming finish to overhaul Bahrain’s 2016 world indoor champion Kemi Adekoya and top the qualifiers in a time of 50.58.
Adekoya, better known prior to this year as a 400m hurdler, was rewarded by trimming 0.14 from her Bahrain record to stop the clock in 50.72 for second to mark her out as a potential danger in the semi-finals.
Natasha Hastings was rewarded for an aggressive run by holding off the late-surging Christine Ohuruogu of Great Britain in heat four.
The US runner caught up the stagger on two of her rivals within the first 100m and had enough in the tank to cross the line in 51.31, 0.09 ahead of Ohuruogu, the 2008 Olympic champion and silver medallist in London four years ago.
It's Miller time
Last year’s world championships silver medallist Shaunae Miller is one of those special talents who can make one-lap sprinting look a ridiculously simple art and the long-legged Bahamian once again demonstrated that ability by cruising to victory in 51.16.
Completely in control, she was able to stride gently down the home stretch, the 1.85m-tall Miller is looking in fantastic shape leading into tomorrow night’s semi-finals. Australia’s Morgan Mitchell finished strongly to grab second in 51.30.
The 18-year-old 2015 world youth champion Salwa Eid Naser showed her ability to mix it with the top senior athletes to hack hefty 0.33 from her Bahrain junior record with a heat six victory in 51.06.
Naser held a clear advantage heading into the home straight and fought off the challenge of the one-two at last month’s European Championships: Libania Grenot of Italy (51.17) and Floria Guet of France (51.29) – to record the third fastest time in first round.
Heat seven lived up to the expected billing as the two athletes competing in the outside two lanes, Shericka Jackson and Kabange Mupopo, dominated.
Jackson, the 2015 world bronze medallist, held a significant lead into the home stretch but despite easing down she still managed to edge out her Zambian rival by 0.03 to take the heat win in 51.73.
In the final heat, Christine Day became the third Jamaican to advance to Sunday evening’s semi-finals by claiming victory in 51.55, also winning by 0.03 from Canadian champion Carline Muir.
Steve Landells for the IAAF