Two-time Grand Slam champion Angelique Kerber reached the quarterfinals of the French Open for the second time by beating Caroline Garcia 6-2, 6-3.
Kasatkina poses a different set of puzzles with her varied game, however, and was quite prepared to engage in long baseline exchanges during Sunday's first instalment, mixing her spins and angles to keep Wozniacki guessing.
They will return on Monday to conclude the battle for the remaining quarter-final place in the bottom half of the draw. She was pregnant when she won the Australian Open in January 2017 for her 23rd Grand Slam singles championship, the most for anyone during tennis' 50-year professional era.
Seconds later, she threw her head back after serving a booming ace that flew past Buzarnescu before she could react.
Winner Kasatkina was asked if she would stick around at Roland Garros to watch the other matches - she said that solving accommodation issues were her priority.
Williams pulled out of the French Open less than an hour before she was supposed to face Maria Sharapova in a fourth-round showdown Monday, the most-anticipated match of the tournament so far.
Keys is yet to lose a set in the French capital and despite her Romanian opponent having beaten Elina Svitolina in the previous round, she was no match for the imposing American.
The other quarterfinal in the bottom half of the draw will match American Madison Keys, the No. 13 seed, against Yulia Putintseva, who now represents Kazakhstan after representing Russian Federation earlier in her career.
The second-seeded German won 4-6, 7-6 (4), 2-6, 6-3, 6-3 against Karen Khachanov, a 38th-ranked Russian, in their first-ever meeting on clay or at a major.
Mouratoglou adds that there was a concern that if Williams tried to play Monday, she would risk making the injury worse and not be able to play at Wimbledon, which begins July 2.
Mouratoglou says: "If it rains, we might gain one more day, and you never know how it feels tomorrow".
Serena Williams of the USA, right, and her sister Venus Williams plays Slovenia's Andreja Klepac and Spain's Maria Jose Martinez Sanchez during their double match of the French Open tennis tournament at the Roland Garros stadium, Sunday, June 3, 2018 in Paris. "Basically, she couldn't play". And while she's "pretty much had every injury in the book", this was the first time dealing with this particular malady.
"I honestly didn't think I played badly this morning", said Wozniacki.
Williams' voice quivered as she said she can't serve because of a problem with her right pectoral muscle.
Instead it was 21-year-old Kasatkina who piled on the pressure in the eighth game, earning a string break points and finally converting the third to take a 5-3 lead.
The onslaught started early for Mertens, who found herself fending off a flurry of break points in her second service game.