Pecker is a key witness in the case against the former US president, who is accused of falsifying business records to cover up hush-money payment
Osaka's 6-2, 7-6 (7/5) victory advanced her into Saturday's championship match against either British eighth seed Johanna Konta or two-time Australian Open winner Victoria Azarenka of Belarus in the same New York COVID-19 quarantine bubble where the US Open will begin on Monday.
The US Open tuneup event is typically played in Cincinnati but was moved in the wake of the deadly virus outbreak.
Two-time Grand Slam champion Osaka said Wednesday she wouldn't play in the semi-finals to protest the police shooting of Jacob Blake in Kenosha, Wisconsin.
"Before I am an athlete, I am a black woman. And as a black woman I feel as though there are much more important matters at hand that need immediate attention, rather than watching me play tennis," Osaka said Wednesday.
"I don't expect anything drastic to happen with me not playing, but if I can get a conversation started in a majority white sport I consider that a step in the right direction."
However, she changed her mind after the tournament rested on Thursday to protest at the Blake shooting.
Osaka, the 2018 US Open and 2019 Australian Open champion, broke for a 2-0 lead and again in the eighth game to take the first set in 38 minutes.
After Osaka broke again for a 2-0 edge in the second set, Mertens rolled through the next four games to seize the lead.
Osaka broke back in the seventh game and held to pull level at 4-4, then battled to the tie-breaker, which she won when Mertens hit a backhand wide.
The Japanese star saved 18 of the 21 break points she faced.
Pecker is a key witness in the case against the former US president, who is accused of falsifying business records to cover up hush-money payment
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