Simone Biles is tipped to win her first Olympic gymnastics gold since 2016 on Tuesday while Paris organizers will be nervously monitoring pollution levels in the River Seine ahead of the men's triathlon.
America's Biles appears to be back at the peak of her powers as she leads the United States on what has been billed as a redemption mission in the women's team final.
The 27-year-old, considered the greatest gymnast of all time, won four gold medals at the Rio 2016 Games.
But she battled the disorientating mental block gymnasts call the "twisties" at the pandemic-delayed Tokyo Olympics three years ago.
Biles dramatically pulled out of most of her events including the team final and the United States settled for silver behind Russia after winning gold in 2012 and 2016.
Biles, a 23-time world champion, went home with a silver and a bronze, and while she won many plaudits for opening up on her mental health, there were critics who accused her of putting herself before her country.
"It has to be for us," Biles said of the United States' bid to recapture their women's team crown.
"It can't be for anybody else. We do it for ourselves and the love of the sport and the love for representing the USA."
The US team topped the rankings in qualifying and Biles wowed an A-list crowd in Paris on Sunday in her first appearance in the French capital, despite tweaking her left calf and competing with her leg taped.
The Seine, which snakes through the French capital, is supposed to stage the swimming leg of the triathlon competition, which begins on Tuesday morning with the men's individual event.
French officials had hoped that holding triathlon and marathon swimming on the river, lined by some of Paris's most famous landmarks, would create enduring images of the Games.
But organisers cancelled a second day of triathlon training on Monday because of the poor water quality and said they would make a last-minute decision on the men's race after reviewing laboratory results overnight.
The waterway was polluted by heavy rain that drenched the opening ceremony on Friday and disrupted the early action on Saturday, leading to discharges of raw sewage.
However, Paris 2024 officials and World Triathlon said they were "confident" that pollution would drop before the start of the competition given the hot, sunny weather, which helps keep bacteria levels down.
Temperatures are due to soar to highs of 35 degrees Celsius (95 degrees Fahrenheit) on Tuesday, prompting weather chiefs to issued a weather warning for Paris and surrounding areas.
On the fourth day of swimming, defending champion Kaylee McKeown of Australia and US world record-holder Regan Smith go head-to-head for 100m backstroke glory.
It is one of three golds up for grabs at La Defense Arena.
The others are in the men's 4x200m freestyle relay and the men's 800m freestyle, in which Ireland's Daniel Wiffen is looking to become the first man from the country to claim an Olympic swimming gold.
The world champion posted the fastest time in the heats on Monday.
The England-born 23-year-old insisted he was not fixated on gold, and with it a place in Irish folklore.
"Any medal's good, it's my first race here, and it's my first time actually being in contention for an Olympic medal, so I'll take any medal, whatever colour," he said.
Gold medals are also up for grabs on Tuesday in fencing, judo, shooting and table tennis.
There is also the final of the women's rugby sevens, after France clinched the men's title in thrilling fashion.
In tennis, the "dream team" of Rafael Nadal and Carlos Alcaraz return to action in the second round of the men's doubles.
