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Barty and Osaka were on track to meet in the last 16 in Melbourne before the latter suffered a three-set loss to Amanda Anisimova.
World number one Barty, who crushed Camila Giorgi 6-2 6-3, always felt there were no certainties despite all the talk about a potential clash against Osaka.
"That was your expectation. My expectation was whoever it would be. I mean, each match is uncertain," she said.
"Each match of tennis, there are no certainties. You have no idea what's going to happen. You just have to navigate your way through as best you can that given moment.
"I have done a good job of that this week. Now it's exciting to get to play Amanda again. We've played before. It will be nice to play each other again in a big match."
Barty and Anisimova will meet for the first time since their 2019 French Open semi-final, which the former won on her way to a maiden grand slam title.
That match was a "turning point" in Barty's career. After losing the first set despite leading 5-1, Barty fell 3-0 behind in the second, only to fight back for a 6-7 (4-7) 6-3 6-3 victory.
"I try to forget, but also, I remember that I learned a lot from that moment," Barty said.
"That was a turning point in my career, and you have to be able to take learnings from those moments, as hard as they are sometimes, and I was able to navigate and find a way through.
"At that point in my life, in my career, it was a massive turning point.
"Obviously it feels like it's a lifetime ago, but some of those memories are still really vivid. Without a doubt we will take that and use that experience, use those feelings and those emotions as best we can come Sunday."