Also on day two of the Paris Games, Rafael Nadal shrugged off injury concerns to set up a second-round showdown at Roland Garros against great rival Novak Djokovic.
After US gymnastics superstar Biles wowed an A-list crowd at a packed Bercy Arena, focus turned to the swimming pool in the evening, with three golds on offer.
Leon Marchand surged to France's first Olympic swimming gold since 2012, obliterating the field to clock the second-fastest 400m individual medley time ever.
With the home crowd roaring on every stroke at La Defense Arena, the 22-year-old led off strongly and never looked back, touching in a new Olympic record time of 4mins 02.95secs, nearly six seconds clear of Japan's Tomoyuki Matsushita.
Torri Huske then beat American teammate and world record-holder Gretchen Walsh to clinch the women's 100m butterfly title, before the clash of the evening in the pool delivered an unexpected twist.
All the talk pre-race was about a blockbuster duel between Britain's reigning two-time champion Peaty and China's world title-holder Qin Haiyang in the men's 100m breaststroke.
They were neck and neck for large parts of a thrilling race, before Martinenghi roared through for victory, pipping Peaty and America's Nic Fink by just 0.02 secs as they shared second.
Qin, who was reportedly among 23 Chinese swimmers who failed drugs tests in the lead-up to the Tokyo Games, only to be cleared to compete, faded to seventh.
Biles wows A-listers
Tom Cruise as well as popstars Lady Gaga and Ariana Grande were among a star-studded crowd watching Biles as the American began her quest for Olympic gymnastics redemption.
The four-time Olympic champion is competing in Paris three years after her campaign at the Tokyo Games unravelled due to a disorientating condition gymnasts call "twisties".
The 23-time world champion, widely regarded as the greatest gymnast in history, entered the arena to a standing ovation and a deafening roar greeted her introduction.
She acknowledged the support with a wave and she blew a kiss to the crowd after her image was flashed up on a giant scoreboard.
Biles opened her qualifying with a sublime performance on the beam, finishing second behind China's Zhou Yaqin.
She tweaked her left calf warming up for her second apparatus, the floor exercise, but still topped the standings.
She also led the vault but narrowly missed out on qualifying for the uneven bars finals.
Biles topped the all-around standings with 59.566 points, while the United States led the way in the team competition.
"It's incredible," said US team technical lead Chellsie Memmel. "What she was able to do looking like she has soreness or something in her leg is remarkable."
Nadal vs Djokovic
Spain's Nadal beat 83rd-ranked Hungarian Marton Fucsovics 6-1, 4-6, 6-4 despite sporting strapping on his injured right thigh and will play top seed Djokovic on Monday.
His meeting with the Serbian 24-time major winner will be the 60th of their storied careers.
"Of course it's beautiful to play against one of the two biggest rivals that I had in my career, especially on this court," said the 38-year-old Nadal, who is at his farewell Olympics.
"But situations are completely different for him, for me. He's being very competitive. I was not being very competitive for the last two years, so in that case, I think probably he is the clear favourite."
On the basketball court, Kevin Durant returned from injury to produce a shooting masterclass as the United States launched the defence of their Olympic crown with a 110-84 win over Serbia.
Durant shot a near-perfect eight-of-nine from the field to finish with 23 points in a convincing win for the reigning champions.
In other sports, Japanese 14-year-old Coco Yoshizawa won women's street skateboard gold in an all-teenage final, seeing off the challenge of compatriot Liz Akama.
