China enjoyed success with a Guan Chenchen and Tang Xijing one-two, but the American gymnast stole the headlines on her first appearance back after withdrawing from the team final last week.
Mental health concerns were cited as the reason for Biles stepping away from the team events and other individual disciplines, though the 24-year-old excelled in her sole event on the beam.
The four-time Rio Olympics gold medallist reminded the world of her quality with a 14.000 score, and, despite Chenchen's last-ditch 14.633, she produced an emphatic return for the United States.
"I was just happy to be able to perform, regardless of the outcome," Biles explained after Tuesday's event. "I did it for me, and I was just proud of myself for being able to compete one more time.
"Just to have the opportunity to compete at the Olympic Games meant the world, because training for five years and then coming here, then kind of being triggered and not being able to do anything, it wasn't fun.
"To go out there and compete one more time and have everyone's support meant the world."
To add to Biles' memorable comeback, the USA's basketball stars nudged past Spain 95-81 to reach the Olympic semi-finals, with Kevin Durrant netting a team-high 29 points to drive his side towards victory.
It was a far from convincing performance from the USA, however, as they trailed 39-25 at one point in the second quarter before rallying impressively to level at 43-43 at the halfway stage.
THOMPSON-HERAH COMPLETES DOUBLE-DOUBLE
Elaine Thompson-Herah motored to sprinting history as she won the 200 metres women's final to complete the sprint double-double.
The Jamaican set a new national record with 21.53 seconds, the second-fastest time in the history of the event, making herself just the second athlete ever, after Usain Bolt, to win both titles at consecutive Games.
Thompson-Herah, who claimed 100-metre gold on Saturday after doubling up for the first time at Rio 2016, just missed out on Florence Griffith-Joyner's 1988 world record by 0.19s.
"Oh my god, it's amazing that I have ever seen this day. That I could complete another double. I can't believe it," the 29-year-old said after the 200 metres final.
"I really had to pull it out to win the 200m. It's a new PB [personal best] and a national record. I am so, so happy.
"Honestly I am so tired, my legs just need some rest. I've done so many races in the last few days, but I am very grateful."
WARHOLM'S WORLD
Karsten Warholm demolished the world record with a remarkable 400 metres hurdles triumph, smashing the previous record run for a second time in five weeks.
At the start of July in Oslo, Warholm cut 0.08 seconds off Kevin Young's longstanding record that was set at the Barcelona Games in 1992, though the 25-year-old slashed even more off in Tuesday's final.
The Norwegian star powered through in 45.94s, hacking an astounding 0.76s off the global mark to secure a memorable victory.
"It's just so big. It's almost like history here. It was the only thing missing from my collection," the 25-year-old said after his win. "I had a World Championships [gold medal]. I had European Championships, I had the world record, the European record.
"The Olympic gold medal is what everybody talks about. I knew this race was going to be the toughest of my life, but I was ready."
The top three finishers all beat the previous Olympic record, with American silver medallist Rai Benjamin running 46.17, yet the performance of Benjamin's life was simply no match for Warholm.
BRAZIL PREPARE TO DEFEND TITLE
Brazil inflicted shoot-out heartbreak on Mexico after a 0-0 stalemate in 120 minutes to set up a final showpiece with Spain.
Reinier converted the crucial spot-kick and, with Eduardo Aguirre and Johan Vasquez missing, the defending Olympic champions ran out 4-1 victors.
"Playing the final match in the Olympics is a dream," said Brazil's head coach Andre Jardine. "The film I saw was the film of life. Everything we’ve gone through. Although we didn’t win in the 90 minutes, we took more risks."
Japan suffered extra-time agony in their semi-final against Spain as Marco Asensio's late winner sunk the host nation to tee up a mouth-watering prospect for Saturday's final.
Despite footballing disappointment, women's boxer Sena Irie provided the host nation reason to celebrate as she edged past the Philippines' 2019 world champion Nesthy Petecio to achieve gold.
Irie's win in the women’s featherweight class final means she is Japan’s first female boxing champion at the Olympics.
CYCLING CHAOS
Germany twice recorded world record rides to earn their first gold – and, in fact, their first ever medal – in the women's team pursuit cycling track event.
A strong Great Britain side, including Laura Kenny, awaited Germany in the final but the team of Mieke Kroeger, Franziska Brausse, Lisa Brennauer and Lisa Klein produced the ride of their lives to finish in 4:06.159, almost two seconds quicker than the previous record.
In a dramatic day of cycling, the reigning world champions Netherlands won their first gold since 1936 in the men's team sprint as Team GB again had to settle for silver inside the velodrome.
That silver for Jason Kenny meant he became the first athlete to win eight Olympic medals in cycling track events and also equalled the total medal haul of Britain's most successful Olympian Sir Bradley Wiggins.
More controversy filled Tuesday's track events, with Denmark's Frederik Madsen crashing into the back of Team GB's Charlie Tanfield inside the closing kilometre of the men's team pursuit heats.
The UCI later deemed the Danes, who were near two seconds ahead of their opponents, to have completed a successful catch to make it through to Thursday's final against Italy, despite the incident.
