Naomi Osaka went into "zombie mode" to reach the third round of the Madrid Open but Karolina Pliskova suffered a shock defeat to Kateryna Kozlova on Tuesday.
Top seed Osaka had to go the distance to reach the last 16, serving up a final-set bagel to beat wildcard Sara Sorribes Tormo 7-6 (7-5) 3-6 6-0 at the Caja Magica.
Simona Halep, who can return to the top of the rankings if she wins the title and Osaka fails to reach the last four, beat Johanna Konta 7-5 6-1.
Pliskova made another early exit, the fifth seed losing to Kozlova 7-5 2-6 6-4.
Angelique Kerber withdrew from the tournament ahead of her match with Petra Martic due to a right ankle injury, but Donna Vekic, Caroline Garcia, Zheng Saisai and Aliaksandra Sasnovich all progressed.
OSAKA 'GOT A BIT LUCKY'
There is certainly room for improvement for Osaka, but the US Open and Australian Open champion struck 46 winners to Sorribes Tormo's eight to advance.
Osaka said: "I feel like in the first set, I got a bit lucky there. And then in the second set, I just got up and then I just got a little bit unfocused.
"And then in the third set, I just, like, went zombie mode, and then just thought of everything that I had to do to win and not necessarily the outcome, but just like the little things in between every game."
Sasnovich, a 6-2 6-2 winner against Anna Karolina Schmiedlova, will be Osaka's opponent in the next round.
HALEP TO DO HER HOMEWORK ON KUZMOVA
French Open champion Halep made only 13 unforced errors and struck 19 winners in a straight-sets defeat of Konta.
The third seed will now come up against Viktoria Kuzmova, conqueror of Julia Goerges and Carla Suarez Navarro, for the first time.
"We haven't played against each other so I don't have the feeling how [Kuzmova is] playing," said Halep. "I will check a little bit, I have enough time.
"It'' a big challenge always when I play with someone that I never played before. So I just want to see what I have to do against her. But the main thing is to focus on myself, on my game, and if I feel it, then I have a better chance."
KOZLOVA UPSETS STRUGGLING PLISKOVA
Having suffered an early exit at the Stuttgart Open before pulling out of last week’s Prague Open on home soil due to a viral illness, Pliskova was short of matches on her arrival in Madrid.
The former world number one had prevailed in three sets against Dayana Yastremska in her opener but was on the wrong end of an upset against Ukrainian Kozlova.
Pliskova was broken for times as the aggressive Kozlova claimed a big scalp after one hour and 51 minutes on court.