Osaka defeated Australian Ashleigh Barty 3-6, 6-3, 6-2, marking her second title in recent times after winning the Pan Pacific Open in Japan last month.
The Japanese world number had to fight back from a set down to overcome her top-ranked opponent in the one hour, 50-minute encounter.
Osaka revealed she’d overcome mental demons to claim the crown.
“Basically I was telling myself to keep fighting even though my attitude was really trash; ‘this is a final, I’m really privileged to be here in the first place, so keep fighting!’” Osaka said.
“I feel like I wasn’t that calm here. I threw my racket once every match. I really wanted to win here, I felt like I had something to prove.”
Thiem also came from a set down to win the men’s singles title over Greek Stefanos Tsitsipas 3-6, 6-4, 6-1.
The triumph marks Thiem’s 15th career singles crown and helps the Austrian reinforce his current world number five ranking.
The match took two hours and 13 minutes, with seventh-ranked Tsitsipas edging a tight first set, before Thiem took over.
“Honestly I think this was one of the best matches I have played so far in my whole career,” Thiem said.
“I think from both of us there were way more winners and unforced errors.”