The stand of Osaka, who eventually won her semifinal Friday but had to drop out of the final with a hamstring injury on Saturday, prompted huge support throughout tennis and far beyond.
But Osaka, 22, settled down and broke to level at 4-4 and then brilliantly saved eight break points to pull ahead 5-4 before displaying exceptional power in the tiebreak.
All three National Basketball Association playoff games at Walt Disney World Resort on Wednesday were postponed amid the calls for change to police brutality and injustice.
Like many athletes in the current political climate, Osaka is illustrating the power of individual protest in changing industries and bringing issues of social justice to a wider audience.
Japan's Osaka, who walked onto the Grandstand court at the Billie Jean King National Tennis Centre in NY wearing a "Black Lives Matter" T-shirt, sealed a 6-2, 7-6 (5) win on her second match point when Mertens put a backhand return wide.
After withdrawing from the tournament, Osaka wrote that the massacre of blacks at the hands of the police is making me sick. "I was scheduled to play my semifinals match tomorrow" (in the Western & Southern Open in New York), she tweeted, but "before I am an athlete, I am a black woman".
"Before I am [an] athlete, I am a Black woman".
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Players, hockey fans, and more have been unsatisfied with the NHL's often eerily silent response to Black Lives Matter . The break gave Varlamov, along with Flyers goalie Carter Hart , plenty of time to rest before Game 3.
Osaka's decision prompted the WTA and ATP to postpone all semi-final matches to Friday, which inspired Osaka to change her mind and play, although she was anxious that other delayed players would be upset.
Osaka said her big personal gains from the move were "confidence and really becoming more aware of the impact my voice could have". I lost to her last time that we played, so I was really looking forward to that.
Azarenka said the title was "special" because it's her first as a mother.
"I totally get her reason 100 percent, so I'm totally supporting her too", Mertens said.
"I just wanted to create awareness in the tennis bubble".
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.
"I don't feel like I'm being fearless. I just feel like - not common sense, but this is what I am supposed to be doing in this moment", she added.