Beijing - US Open champion Naomi Osaka said that she apologised to beaten opponent Zhang Shuai for her emotional behaviour in their roller-coaster China Open quarter-final on Friday.
The 20-year-old Japanese fought back tears - and rescued a match that was slipping away from her - to defeat the unseeded home hope 3-6, 6-4, 7-5 to reach the Beijing semi-finals.
Osaka, who looked glum even in victory and had flung her racquet around during the match, embraced Zhang at the end.
"I was basically apologising because I feel like today I was more emotional than what I normally am trying to be like," said the world number six.
"The match itself was very draining. I feel like we both fought as hard as we could."
Asked at the post-match press conference - where she again looked unhappy - why she had had tears in her eyes on court, she replied curtly: "Because it was a hard match. That's basically why."
Osaka faces Latvia's unseeded Anastasija Sevastova in Saturday's semi-finals on Beijing's outdoor hard courts.
The 45th-ranked Zhang - with the crowd on her side - raced into an early lead against an unusually erratic Osaka, who had won 24 of her previous 28 games.
The eighth seed, who stunned Serena Williams to win her maiden Grand Slam last month in New York, belatedly began finding some range.
But she continued to rack up errors and conceded the first set when she hit into the net. She was to make 66 unforced errors in all.
The 29-year-old Zhang, who has won two career WTA singles titles, saved six break points to take the lead in the second set.
Zhang broke to go 2-0 up and Osaka - who has admitted struggling with the fall-out from the US Open final, when Williams rowed with the chair umpire - began welling up.
Osaka, who has rocketed to worldwide sporting attention since her US Open feat, said that she had felt sluggish.
"I just kind of felt a little bit tired practising and warming up for the match," she said.
"Then when I stepped on the court, I just felt a little bit drained."
Men's top seed Juan Martin del Potro had no such trouble, bulldozing his way into the semi-finals with a 6-3, 6-0 demolition of Filip Krajinovic of Serbia.
The towering Argentine, enjoying one of the best seasons of his injury-blighted career, will meet in-form Italian Fabio Fognini in what promises to be a far tougher test.
Fourth seed Fognini continued his march towards a fourth title of the season with a 6-4, 6-4 victory over Marton Fucsovics of Hungary.