Heat 2 was much more pedestrian but also more exciting producing a blanket finish with the first five separated by only 0.16 of a second. Sifan Hassan won in 4:08.89 with Jenny Simpson and Laura Muir second and fourth in a heat where only the top six qualified.
Olympic Champion Faith Kipyegon won the third heat in 4:03.09, with the top six all running 4:03.60 or better. Britain's Sarah MacDonald was ninth in a PR of 4:05.48 but in a fast race ninth was good enough to get her through to the semis.
So what did we learn from the heats?
All the fancied runners made it through to the semis. While I have described times and tactics all that is in a sense irrelevant as it is all about qualifying.
We learned that Faith Kipyegon, the dominant runner at the distance in 2016, still looks strong. Tactician extraordinaire, Jenny Simpson, just did what she needed to make progress. Genzebe Dibaba ran the fastest time in the heats to show she is in shape. Caster Semanya dispelled any doubts about her ability to be competitive at the longer distance, displaying her awesome finishing power. Laura Muir did what she needed to. While she has been strongly tipped for a medal, it will be a tall order for her to make the podium in what looks to be one of the most competitive events of the program.
This is what some of the runners thought of their performance:
Genzebe Dibaba: "I'm in a good condition - last year I had injuries but yes I am good now."
Jenny Simpson: "The thing about the world championships for Jenny Simpson is that I have done well here when I was supposed to do well. The world championships bring out the best in me."
Faith Chepngetich Kipyegon: "The race was good, I'm so happy to qualify for the semifinal tomorrow. I know in a championship there is no pace-maker, so the race can be tactical rather than quick. I do not mind which it is though."
Laura Muir: "It was brilliant out there on the start line. In terms of major events, being at Glasgow 2014 helped and doing a few rounds at Belgrade was good so I'm lucky I've got a few championships under my belt now. To have that support out there was great. It feels like a real positive".