In one regard, Serena Williams still has a ways to go to be like best friend Caroline Wozniacki, and it raised speculation about the top-ranked tennis player's health.

Williams on Sunday played host to her Serena Williams Live Ultimate Run, a charity race in Miami, Fla., People.com reported.

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The event raises money for the Serena Williams Fund, which People reported focuses on "helping individuals and communities affected by senseless violence, and promoting equal access to education."

Participants can choose from three difference races -- a quarter marathon, a 5K run and an "ultimate kids dash."

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Williams recently returned to training after skipping the last two months of 2015 WTA season after her stunning U.S. Open semifinal loss to Roberta Vinci that ended her dreams of winning the calendar Grand Slam.

She announced in a video on her Facebook page that she was unable to complete her run. She apparently entered the 5K run, and she said she could only run part of the way and had to take a taxi to the finish line, according to vulture.com.

"I didn't run the whole way this year, but next year I will," she said on her Facebook post.

Last year, Serena was a spectator as Wozniacki ran the New York City Marathon. Certainly, Williams could be forgiven for not competing the quarter marathon portion of her event, but a 5K seems as if it shouldn't be that difficult a task, even with all of Williams' time off. Her inability to complete the race led People to speculate that the knee injury she said plagued her during her U.S. Open run may have been a factor.

"It appears Serena Williams' injuries, which led her to withdraw this fall from multiple competitions in order to heal, may still be plaguing her," People reported.

Indeed, if Williams still is recovering from a balky knee, running distances may not be the best thing for it. In typical Williams fashion, she did not say why she didn't finish the race.

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