Top seed Victoria Azarenka overcame an early call for the trainer to beat Roberta Vinci and reach the quarter-finals of Tokyo's Pan Pacific Open.
The world number one appeared to be feeling unwell at 3-3 and had treatment on court, but came out firing.
Azarenka raced to a 6-4 6-2 win to set up a quarter-final against Germany's fifth seed Angelique Kerber.
Second seed Maria Sharapova, who
beat Heather Watson
in the previous round, saw off Lucie Safarova 6-2 7-6 (7-5).
- Victoria Azarenka (1st seed) v Angelique Kerber (5)
- Sara Errani (6) v Nadia Petrova (17)
- Agnieszka Radwanska (3) v Caroline Wozniacki (10)
- Sam Stosur (8) v Maria Sharapova (2)
"After the long match yesterday, I'm very happy to be able to win today," said the Russian, who won the title in 2005 and 2009. "I was a little bit down in the second set and I'm just happy to win in two sets."
Sharapova next plays Australian eighth seed Samantha Stosur, who beat 12th seed Dominika Cibulkova 6-4 7-5.
In the final match of the day, Azarenka received courtside medical attention, which included a blood pressure test midway through the first set, but recovered to win the next three games and with it the set.
"I felt dizzy on the court and my head was spinning in between points, which made it difficult to focus," she said.
"What exactly it is, I don't know. I am still trying to figure it out. Hopefully, I'll know as soon as possible and get rid of it."
Third seed Agnieszka Radwanska will meet 10th seed and former world number one Caroline Wozniacki in an intriguing quarter-final.
Poland's Radwanska brushed aside Jamie Hampton of the US, 6-4 6-3, while Dane Wozniacki battled past China's seventh seed, Li Na, 4-6 6-3 6-4.
"After the first set of my first match, my energy level dropped, but since then I've been feeling great," said Wozniacki, the 2010 champion.
"Today Li Na was playing very close to the baseline and pushing me away, taking the balls very early - she was all over me. But in the second set I really started to step in and push her away."
In Beijing, British number one
has been drawn against Spain's Garbine Muguruza for Friday's first round of qualifying at the China Open, while
will play Chinese wildcard Ran Tian.