TOKYO, Japan - Former WTA World No.1 Caroline Wozniacki had gotten the upper hand on longtime rival Dominika Cibulkova when the Slovak was forced to retire due to a right thigh injury, trailing the Dane, 3-6, 7-6(5), 3-1 in the Toray Pan Pacific Open quarterfinals.

Wozniacki became the first woman to win 50 matches in 2017 en route to the last eight, but appeared in trouble as she fell behind a set to Cibulkova, who had beaten her in their most recent encounter 18 months ago at the St. Petersburg Ladies Trophy.

Cibulkova, who was ranked as high as World No.4 earlier this year, appeared keen for her first big win of the season after finishing runner-up at the Connecticut Open presented by United Technologies, breaking to love late in the second set and holding two match points serving for the upset.

The No.3 seed saved both - the second with a thunderous backhand down the line - and took a tense tie-break that only featured one mini-break to level the match.

Cibulkova looked increasingly hampered by her right thigh, leaving court after dropping serve early in the decider to get it strapped, but ultimately couldn't continue after two hours and 27 minutes on court.

“I thought we both played really well - it was definitely exciting tennis,” Wozniacki said after the match. “She had match points in that second set, somehow I managed to pull it out and turn the match around.

“I just kept going for it and going for the lines. She was playing really well so after that point [the match points] I just had to go all out.”

By match's end, the stats showed positive numbers for both women, with Cibulkova striking 47 winners to 41 unforced errors, while Wozniacki turned around a disappointing first set differential to finish with 10 more winners than errors (33 to 23).

Up next for the World No.6 is top seed and new WTA World No.1 Garbiñe Muguruza, who faced fewer problems against No.9 seed Caroline Garcia earlier in the day.

"It’s going to be a very different match," Wozniacki said. "You just gotta look forward to it and recover and be ready to give everything again."