Sifan Hassan began her shot at an unprecedented long-distance treble by winning the women's 5000 metres, while Jasmine Camacho-Quinn made up for her Rio heartbreak in the 100m hurdles on Monday.
Elaine Thompson-Herah boosted her hopes of breaking new ground herself, while Valarie Allman ended the reign of Sandra Perkovic in the discus on a soaking night at the Olympic Stadium in Tokyo.
Here's a round-up of the best action of Monday's athletics events.
HASSAN'S HAT-TRICK HISTORY BID OFF TO A FLIER
Hassan is aiming to become the first athlete to win a 1500m, 5000m and 10,000m treble at a single Games and began her bid for that unprecedented achievement by taking out the second of those events.
Having recovered from a late fall to get through the 1500m heats in the morning, the Ethiopia-born Dutchwoman finished her double duty on Monday with a winning time of 14:36.79.
It was Hassan's trademark blistering finish that won the day as she made her move with 300m to go and pulled clear down the straight. Hellen Obiri and Gudaf Tsegay were second and third respectively.
REDEMPTION AND HISTORY FOR CAMACHO-QUINN
Five years ago in Rio, Camacho-Quinn endured the heartbreak of crashing out of the semi-finals after hitting a hurdle.
But, after breaking the Olympic record in the semis here on Sunday, she became the first athletics gold medallist for Puerto Rico and only the second in any sport after Monica Puig by running a 12.37s.
World-record holder and main rival Kendra Harrison took second for the United States ahead of Megan Tapper of Jamaica.
"Everything happens for a reason," she said. "I came through with the gold, my first gold medal. For such a small country, it gives little people hope. I am just glad I am the person to do that."
In the women's 400m hurdles semis, Femke Bol – one of the medal favourites – made it through, with the quickest time posted by Sydney McLaughlin.
THOMPSON- HERAH SETS 200M STANDARD
Thompson-Herah is aiming to become the first woman to complete a double-double (winning the 100m and 200m at the same Games twice).
She was quickest in the 200m semi-finals with a time of 21.66s, while Shelly-Ann Fraser-Pryce was fastest in her heat with a 22.13s.
Wayde van Niekerk's hopes of defending the 400m men's title are over after he crashed out of the semis. Kenya's Kirani James was fastest with a 43.88s.
ALLMAN DETHRONES PERKOVIC
Perkovic became only the second woman to win consecutive Olympic discus titles with her triumphs at London 2012 and Rio 2016 but her reign was ended by US champion Allman.
The soaking conditions made life tricky in the women's final and there had to be an interruption due to the rain.
But it was Allman, who threw furthest in qualifying, who was left celebrating with a huge throw of 68.98.
Kristin Pudenz earned silver for Germany with a personal best of 66.86m, while Yaime Perez was third for Cuba. Perkovic finished outside the medals in fourth.
EL BAKKALI CHASES THE DREAM, TENTOGLOU SAVES THE BEST FOR LAST
In a pouring wet evening session, Morocco's Soufiane El Bakkali made up for the heartbreak of finishing fourth at Rio 2016 by winning the men's 3000m steeplechase in a time of 8:08.90 ahead of Ethiopia's Lamecha Girma, who took silver.
Athletes from Kenya had previously won this event at all but two Games since 1968, and both were Olympics boycotted by the nation. Benjamin Kigen finished third here.
Earlier in the day, Miltiadis Tentoglou – the 2018 European champion – saved his best for last by winning the men's long jump with his sixth and final effort.
The Greek's effort of 8.41m matched that of Cuba's Juan Miguel Echevarria but he was bumped into gold due to his second-best distance of 8.15m. A disappointed Echevarria was consoled by countryman Maykel Masso, who finished third.
"What an incredible competition," Tentoglou said. "What an incredible jump, the last jump. I wasn't able to get it right at the start. But in the end I managed to pull something out to get the medal."