Laura Muir warned she is saving her best for last after launching her bid for Olympic glory.
The Scot clocked four minutes 03.89 seconds in Tokyo on Monday to reach the 1500 metres semi-finals.
Muir, the European champion, came second in her heat behind Canada's Gabriela DeBues-Stafford but insisted there was plenty in the tank.
"You don't want to have any disrespect to any of the girls out here, but I want to save as much as I can for the final," she said ahead of Wednesday's semis.
"It's gone as smooth as it could be – I've been out in Japan for a couple of weeks now so feeling really prepared and it's really good.
"It didn't feel that fast so that's good. I just wanted to qualify for the next round as comfortably as possible. So that felt really good out there today and I am looking forward to the semi-final."
Rival and favourite, the Netherlands' Sifan Hassan, fell at the start of the final lap in her heat but still managed to win after overhauling the field in a stunning final 350m. Hassan is attempting to win the 1500m, 5,000m and 10,000m.
Team GB's Katie Snowden also progressed in four minutes 02.77secs but Revee Walcott-Nolan missed out by 0.01s.
Greece's Miltiadis Tentoglou took the men's long jump title with a leap of 8.41m after world champion Tajay Gayle pulled out injured while Jasmine Camacho-Quinn of Puerto Rico claimed gold in the women's 100m hurdles.
In the women's 200m – without Team GB star Dina Asher-Smith after she pulled out following her battle with a hamstring injury – 100m champion Elaine Thompson-Herah progressed to the semi-finals.
Shelly-Ann Fraser-Pryce, who claimed 100m silver, won her heat in 22.22s but Shericka Jackson missed out after the sprinter completed a Jamaican clean sweep in the 100m on Saturday.
Great Britain's Beth Dobbin ran a season's best of 22.78s to reach Monday evening's semis.
She said: "I can't ask for much more than that, I ran the bend how I wanted to and the straight felt really controlled. There's more in the legs later.
"I could see I was in contention so I tried to stay relaxed. There's a couple of tenths from that run. I need to see how I recover.
"It's one of the most stacked 200m I have seen for a long time and it's missing a few names.
"I walked for the bus at 7.30am and it was blisteringly hot, I'm from up north – we're not used to this weather."
