Halep said afterward she felt a "little bit sick" and plans to "sleep all day" on Friday.
Fifth seed Alexander Zverev, ninth seed Fabio Fognini and 13th seed Borna Coric all progressed easily although Juan Martin del Potro, the eighth seed, needed five sets to subdue Japan's Yoshihito Nishioka.
The second set proved a much closer affair, taking until the fifth game for a serve to be broken as Azarenka went 3-2 up.
After Azarenka took almost 11 minutes to return to the court for the third and deciding set, she wasn't able to convert on potential break points, and Osaka held serve in the first game. "You know, against Vika, she has the ability to hit winners".
"I actually played well today", Osaka said.
Her ability to apply fixes during matches catapulted her to two Grand Slam titles and the World No. 1 ranking in less than a year. "It will be the third match in a row that I get to play against someone that I have never faced before in a match".
"She's been in those situations where those margins are really small", said Azarenka. The 21-year-old tennis star will join the ranks of Steffi Graf and Margaret Court if she wins the French Open 2019 and her third consecutive Grand Slam in a year.
This time, Osaka ceded the first four games against Azarenka and was responsible for their match's initial seven unforced errors. That kind of just relaxed me.
Siniakova, the world No. 42, also endured a marathon win, putting out Greek 29th seed Maria Sakkari 7-6 (7-5), 6-7 (8-10), 6-3 in a three-hour 10-minute duel. But on her third attempt at match point, Osaka finally converted, and eluded the upset in nearly three hours of world-class tennis. "I need to have some more of those experiences".
Asked about it after the win over Nara, she said: "I'm not going to get into that".
Osaka remains in line for quarter-final match up with Barty, who is on the verge of breaking into the top five after her 7-5 6-1 demolition of American Danielle Collins.
Second seed Pliskova was the last of the four players who had a shot at overhauling Osaka left in the draw when she stepped onto Court Philippe-Chatrier to face Petra Martic on Friday.
"To be honest, it's a good choice against players like us who stand far behind the baseline", said Thiem.
The eighth seed was made to work for his place in round three and eventually saw off his Japanese rival 5-7, 6-4, 6-2, 6-7, 6-2 in a contest that lasted 3 hour 46min.