The 15-year-old excelled in the Rod Laver Arena and won 6-3 6-4.
Osaka won the pair’s only previous meeting at last year’s US Open, but it was Gauff who was victorious in Melbourne, winning in 67 minutes.
“I don’t know where that came from,” the American said after the match. “Honestly, like, what is my life?”
“Two years ago I lost in the first round in juniors and now I’m here,” Gauff added.
“I was telling myself one point at a time and keep fighting. You never know what happens on this court.”
Gauff will play either China’s Zhang Shaui or American Sofia Kenin in the fourth round.
15-year-old Coco Gauff dethrones #AusOpen champion Naomi Osaka, 6-3, 6-4 to move into the second round.
She becomes the first ever teenager to defeat a Grand Slam champion in Week 1 of any majors event. Future is now. pic.twitter.com/JN6JqQVZmJ
— Kristoffer Ed Bellen (@KristoffBellen) January 24, 2020
'Two years ago I lost first round of juniors. Now I’m here.
This is crazy. Like, what is my life?'
— Live Tennis (@livetennis) January 24, 2020
Coco Gauff SHOCKS defending #AusOpen champ Naomi Osaka 63 64 to reach the 4R
She played like *she* was the No3 seed. Mature, aggressive, collected tennis.
But cmon, this is BONKERS. Well played Coco pic.twitter.com/Snj41aEtuf
— Nick McCarvel (@NickMcCarvel) January 24, 2020
15-year-old Coco Gauff, who has been soaring up through the tennis ranks at a massive speed, jumps even higher with the biggest win of her career, by far:
6-3, 6-4 over defending #AusOpen champion Naomi Osaka.
Coco Gauff, into the fourth round, will play Zhang or Kenin.
— Ben Rothenberg (@BenRothenberg) January 24, 2020
Coco Gauff knocks out defending champion Naomi Osaka!
She defeats 2 time Grand Slam Champion Naomi 6-3 6-4
— The SportsGram (@TheSportsGram) January 24, 2020
The way Coco Gauff knocked off Osaka in straight sets ??
— ACE Boogie (@TheFitOne_21) January 24, 2020
Coco Gauff is the new black woman tennis sensation. She's overtaken Madison Keys and Sloane Stephens and is soon taking over from the established names.
— Nner® (@Jarnner) January 24, 2020
Defending champion Naomi Osaka falls to 15-year-old Coco Gauff. The American tennis prodigy stunned Osaka with a 6-3 6-4 victory to reach the fourth round of the Australian Open. This victory sets up Gauff to interesting meeting with either Sofia Kenin or Zhang Shaui from China.
— Jamagn (@jamagning) January 24, 2020
Wow amazing! I root for both of them, but I'm really glad Coco Gauff won and is moving on. https://t.co/2VYtJNZcUz
— Laaaa (@Kingcam) January 24, 2020
Steady, aggressive, and with ice running through her veins.
Here’s how Coco Gauff turned the tables on defending champion Naomi Osaka to score her 1st Top 5 win and book her 2nd Slam Round of 16. #AusOpen
— Olamide Abe (@MrJoyoung) January 24, 2020
— Jayden Borowiecki™ (@j7bo_) January 24, 2020
Teen sensation, Coco Gauff has beaten the defending Champion Naomi Osaka to reach the 4th round.
In her words “Two years ago I lost first round in juniors, and now I’m here. This is crazy.“
— ?? NEFERTITI – (The beautiful one has come)?? (@firstladyship) January 24, 2020