Taking on the experienced French 16th seed, who is an aggressive baseliner like Osaka, was always going to be a big challenge for the twice Australian Open champion, who returned to tournament tennis only two weeks ago after 15 months out.
The power that took Osaka to four Grand Slam titles before she took time off to have daughter Shai was still there but it was always going to take the 26-year-old time to rediscover her accuracy in clutch moments.
A double fault gave Garcia the first break point of the match in the fifth game and the 30-year-old former world number four converted it to edge ahead. Osaka had a sniff of a break back at deuce in the eighth game but dumped a routine backhand into the net and Garcia held with back-to-back aces before going on to take the set.
The Japanese former world number one won her first three service games of the second set to love, but Garcia's serving was brilliant and Osaka was unable to put any pressure on it.
Osaka had beaten Garcia 6-2, 6-3 on the way to her 2021 triumph at Melbourne Park but this was a much tighter contest with the second set decided in a tiebreak.
Still hitting the mark with her first serves, Garcia raced to a big lead and clinched the win on her first match point when Osaka dumped another backhand into the net.
Afterwards, the French player said she had a huge amount of respect for Osaka and was sure it would not be long before the Japanese former world number one was back at the top of the game.
"She has had an amazing career, she has been away for 15 months and been through a lot," the 30-year-old said.
"I hope she can enjoy her tennis. She has done a lot for tennis in the past few years. Six months after giving birth and she's playing amazing, so we have to watch out."
"I had to be as solid as I could and be aggressive every time I had the chance," she added.
"She was really trying to put pressure on my second serve but I managed to stay solid and be a bit more aggressive at the end."
It was Osaka's earliest exit in eight visits to the Australian Open and her third successive loss in the opening round of a Grand Slam after she fell at the first hurdle at the French and US Opens in 2022.
I thought it was a really good match. For me, I felt like I did the best that I could possibly do," she told reporters.
"I think I still feel like a bit disappointed... I did feel nervous, but I felt like I kept telling myself to be positive. I thought I kept doing better as the match went on.
"So, yeah, I'm not too happy right now, but I think I can learn from the progress."
The 26-year-old was clearly struggling to reconcile her disappointment at the loss with the fact that she was so competitive after such a long period on the sidelines.
"Of course, there's a voice in my head that is, 'Who are you to think you can come back and immediately start winning matches?'," she said.
"I don't know. I kind of always expect myself to stand a chance anyways."
