Great Britain's Laura Muir said she is "more prepared than ever" after easing through the Tokyo 2020 1500m heats in her pursuit of a first Olympic medal.
Muir clocked four minutes 3.89 seconds to finish second. Six athletes from each heat progress to the semi-finals.
In a dramatic moment, world 1500m champion Sifan Hassan fell with 400m to go but won her heat in 4:05.17.
Great Britain's Katie Snowden qualified sixth in her heat in 4:02.77 but Revee Walcott-Nolan came seventh to miss out.
Walcott-Nolan's 4:06.23 was a PB but left her seventh in the list of fastest losers, with six going through. Kenya's Faith Kipyegon - the Olympic champion - clocked the quickest time overall with 4:01.40.
Muir finished seventh in the 1500m final at Rio 2016Muir, though, is Great Britain's major medal hope in the event and, having decided not compete in the 800m, her Tokyo campaign started with a routine heat as she came home behind Canada's Gabriela Debues-Stafford.
"It's really nice to get that first run out of the way and feel comfortable," said Muir, who was seventh at Rio 2016 and fifth at the 2019 World Championships.
"I am more prepared than ever. I have improved so much. I am in the best condition I could be and hopefully any situation I can deal with."
Greek gold and a Jackson error
Juan Miguel Echevarria was left beating the ground when injury hampered him as Miltiadis Tentoglou won long-jump goldIf Muir's progression was straightforward, Hassan, who is due to compete in the 5,000m final later on Monday, was made to work in the final 300m so she could make up ground and take on the field after her fall.
Ethiopia-born Hassan - who runs for the Netherlands after arriving in the country as a refugee aged 15 - is expected to challenge for a medal over 1500m, 5,000m and 10,000m in Tokyo.
In the women's 100m hurdles final Jasmine Camacho-Quinn won Puerto Rico's first ever track and field gold by powering home in 12.37 seconds, edging out America's Kendra Harrison (12.52) and Jamaica's Megan Tapper (12.55).
"For such a small country, to give little kids hope, I'm just glad I'm the person to do that," Camacho-Quinn said afterwards.
In a dramatic final round of the men's long jump final, Greece's Miltiadis Tentoglou used his last effort to jump 8.41m.
The mark tied that of Juan Miguel Echevarria but Tentoglou took gold by virtue of his second-best jump on the day being longer than that of his Cuban rival.
Echevarria had one more chance to leap to gold but pulled up with a hamstring problem on his run up.
In the women's 200m heats Jamaica's Shericka Jackson - who won bronze in the 100m - finished fourth when she sensationally eased down too soon, missing out on the qualification places by four one-thousandths of a second.
Beth Dobbin - Great Britain's only 200m hope after Dina Asher-Smith withdrew with injury - qualified in second in her heat in 22.78 seconds.
Fancied names such as Ivory Coast's Marie-Josee Ta Lou, Bahamas' Shaunae Miller-Uibo, Jamaica's Shelly-Ann Fraser Pryce and Dafne Schippers of the Netherlands all progressed.
