Above all, she defended her serve effectively, saving all seven points she faced, all of which came in the first set. Serving for that set at 5-4, Siniakova came back from a 0-40 deficit to hold, saving the first break point with a deft backhand drop-shot winner. She broke Osaka’s forceful serves three times.
But Saturday's result was hardly all about Siniakova's cleverly executed game plan, fighting spirit and world-class backhand.
Osaka made errors by the baseline: 38 unforced errors in all, compared with Siniakova's 13. Osaka repeatedly struck the net cord with huge groundstrokes and service returns. By the end, she was missing by much larger margins, as Siniakova reeled off the last five games for the first time.
Osaka showed flashes of fine form in her three clay-court events before the French Open, and she started through her first two rounds here after very slow starts to defeat Anna Karolina Schmiedlova, 0-6, 7-6 (4), 6- 1, and the former No. 1 Victoria Azarenka, 4-6, 7-5, 6-3. But a third comeback win was not to be.
"Today, I felt very tired," Osaka said. "And like the other matches too, I had this headache, right, but I didn't feel tired. I know my head is just my stress kind of. "
But Osaka also gave credit to Siniakova.
"She played great," Osaka said. "I am not going to keep saying that I was tired, because obviously she was playing a super-long match her last round, and she's tired too probably."