- Ben Reed / ISA

After a day full of big performances and big upsets, the Quarterfinalists and have been decided and the first-ever Olympic Finals Day is locked in.

Stephanie Gilmore -- who has the most World Titles of any athlete in the field and was the form surfer on Day One -- was knocked out in Round Three by South Africa's Bianca Buitendag.

With plenty of swell in the water and some mixed up conditions for Round 3, Gilmore, a clear medal favorite, was left in need of a second good score against Buitendag.

"I had five minutes to try and get a score back but couldn't manage to make it happen," Gilmore said. "Total frustration … that's just the nature of surfing, we're all dealing with the ocean and sometimes the waves come, sometimes they don't. You've just got to accept that," she said.

Steph Stephanie Gilmore was a clear favourite for a medal given her seven World Titles - Sean Evans/ISA

And the upsets kept coming in quick succession at Shidashita Beach. France's Johanne Defay -- another medal favorite -- was defeated by Portugal's Yolanda Hopkins Sequeira, and soon after Tatiana Weston-Webb, another top seed, was eliminated by Japan's Amuro Tsuzuki.

Hopkins, Buitendag and Tsuzuki yesterday had to surf through the repechage round to keep their runs alive, and then all came back on Day Two to eliminate top seeds.

Meanwhile, heat two of the men's Round Three came with guaranteed heartbreak built in. Team USA's John John Florence and Kolohe Andino faced each other, with Andino taking the win.

Both these athletes fought their way back from injuries that required surgery in the months before the Games, each putting in a momentous rehabilitation effort to even be able to surf in Japan.

Why were they up against each other? Florence had to surf through the repechage round, and this affected his seeding, creating a draw straight out of Team USA Coach Brett Simpson's worst nightmares. Andino knew he had to bring it, going up against his two-time World Champion teammate. He began his heat with a big air reverse, and backed it up with another in the final five minutes to lock in the second-highest heat score of the day, a 14.83.

Kolohe Andino Team USA's Kolohe Andino and John John Florence met in Round Three - Sean Evans/ISA

"John and I are very honored to compete for the USA, and whether it was ankle slappers or big standup barrels, it was going to be an intense heat and really fun to watch - one for the history books," said Andino.

"I haven't done a maneuver like that in 7 or 8 months. I surprised myself," he added.

"Kolohe and I surf against each other on the tour all the time, so I was stoked when I saw the draw. It's a fun challenge, a fun battle," Florence explained. "I would have loved to win a Gold Medal, but I am pretty happy just being here and surfing against the best in the world. I am going to take what I can from my experience and learn from it. Hopefully I will be here next time. I would love to see surfing in many more Games in the future."

Caroline Marks Caroline Marks secured the highest two-wave total on Day Two - Ben Reed/ISA

But it was another USA team member who posted the best two-wave total on Day Two, with Caroline Marks locking in a 15.33 in her match-up against Japan's Mahina Maeda.

"I am stoked that we have a lot more size and opportunities today," said Marks. "I've definitely thought about the runway of Olympic Games in my future, but right now I am just trying to enjoy this moment in history. It's so cool to be a part of this. If I can be in more Games that would be amazing."

Teammate Carissa Moore also advanced today, and given the list of big names surprisingly eliminated, they are now squarely the medal favourites in the women's draw.

On the men's side, it's Team Brazil who is looking almost unstoppable. Australia's Julian Wilson and Brazil's Gabriel Medina took their match-up right to the buzzer. Wilson, needing an 8.60, launched an air in the final seconds but came up short, allowing the Brazilian to move on to Finals Day.

Regardless of the result, it was one of the most intense and exciting heats seen in this Games so far, followed closely by Italo Ferreira's victory against New Zealand's Billy Stairmand.

And when it comes to Team Australia, while Wilson and Gilmore have been eliminated earlier than many would have expected, Fitzgibbons is still in the mix, and Owen Wright put in one of the best performances of the event so far, earning a 15.00 two-wave total against France's Jeremy Flores.

Conditions are set to improve on Tuesday (local time), after which point the swell is forecast to drop off sharply -- meaning there's a strong chance the event could wrap tomorrow, with surfing's first-ever Olympic medallists decided.

Men's Olympic Quarterfinals Matchups
Kanoa Igarashi (JAP) vs. Kolohe Andino (USA)
Michel Bourez (FRA) vs Gabriel Medina (BRA)
Italo Ferreira (BRA) vs. Hiroto Ohara (JAP)
Luccas Mesinas (PER) vs. Owen Wright (AUS)

Women's Olympic Quarterfinal Matchup
Bianca Buitendag (RSA) vs. Yolanda Hopkins (POR)
Brisa Hennessy (CRC) vs. Caroline Marks (USA)
Carissa Moore (USA) vs. Silvana Lima (BRA)
Amuro Tsuzuki (JAP) vs. Sally Fitzgibbons (AUS)

World Surf League
Download it for free on the App store. Download it for free on Google Play.