Williams has said she doesn't know how to define her legacy, but it is all around, whether embodied by players who credit her with being an inspiration, such as four-time major champion Naomi Osaka or French Open runner-up Coco Gauff, or in rules changes that clearly, or at least likely, are a product of episodes involving her.

"Her legacy is really wide, to the point where you can't even describe it in words. She changed the sport so much. She's introduced people that have never heard of tennis into the sport. I think I'm a product of what she's done. I wouldn't be here without Serena, Venus, her whole family. I'm very thankful to her," Osaka said.

"I also was trying to figure out how to sum it into words. I honestly think that she's the biggest force in the sport."

A line can be drawn to the decision this year by the US Tennis Association to allow in-match coaching for women and men at a Grand Slam tournament for the first time from the chaotic 2018 US Open final in which Williams ended up being docked a game after being warned about receiving instructions from her then-coach, Patrick Mouratoglou, during a loss to Osaka.

The proliferation of electronic line-calling, to the point that there are no line judges at US Open matches anymore, can be traced back to a 2004 quarterfinal match at Flushing Meadows in which multiple erroneous rulings went against Williams during a loss to Jennifer Capriati.

At the US Open alone, there were other run-ins with officials (who can forget the foot-fault brouhaha in her 2009 semifinal against Kim Clijsters), groundbreaking fashion choices (a catsuit in 2002; knee-high boots two years later) and plenty of triumphs, dating all the way back to 1999, when a 17-year-old Williams beat Martina Hingis for her initial Grand Slam trophy.

So the Ashe court provides a fitting backdrop for a farewell, although Williams did not quite explicitly say that she would never compete again after the US Open while telling the world via an essay in Vogue magazine that she was prepared to begin "evolving away from tennis" to focus on having a second child and pursuing her business interests. Every time she steps on court in New York will be treated as if it might be the last time. Even her practice sessions have been attended by throngs of fans in the days prior to the tourament's start.

"Who knows if there's going to be another Serena again? I doubt it," said Kovinic, who is ranked 80th this week and never has been past the third round at a major tournament.

"I'm honored to have this chance to play her." It will be only the fifth singles match for Williams over the past 12 months, because the American was off the tour from a first-round injury at Wimbledon last year until a first-round loss there this year.