The 13th seed has spent years taming her own tendency toward self-recrimination, and showed the best evidence of her new career outlook as she reclaimed her composure with a bathroom break before the third set at Arthur Ashe Stadium.
"The match was definitely really tough. I don't know, I just felt like I had to fight," said Osaka, who will next play Belgian Elise Mertens.
"I think I was a little bit too hard on myself, so I took the bathroom break, tried to regroup and have no regrets."
The winner in 2018 and 2020 was pristine through the first set, with just four unforced errors, as she broke her opponent to love in the fourth game. The Czech did herself no favours with three double faults in the eighth game before letting Osaka break again on set point with an unforced error.
It was a different story in the second set, where Osaka's form dropped just as Siniakova hit her stride, and the Japanese 13th seed put up 20 unforced errors that gave the Czech too many chances.
After trading breaks twice in the second set, Osaka tapped her racket on the court in frustration after whacking a backhand into the net and handed Siniakova the break with another backhand error in the penultimate game.
Osaka went to the locker room before the third set, a decision that helped her move on.
"I just told myself that it's an honour to play on this court," said Osaka, who reached the semi-finals a year ago. "I just want to keep playing in front of you guys, if possible."
She emerged back on the court with new resolve, sprinting through the first three games and saving two break points in the fifth before closing it out with an unreturnable serve.
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