Tennis superstar Serena Williams showed her support on Friday for former NFL players Colin Kaepernick and Eric Reid's national anthem protests.

Williams -- the winner of 23 Grand Slam titles -- discussed their efforts after she defeated her sister, Venus, at the US Open in Flushing, New York.

Kaepernick and Reid, former teammates with the San Francisco 49ers, were in attendance at the Williams sisters' match and received large ovations when each was shown on the stadium's big screen.

Williams said that everyone should "be completely grateful and honored" at how both men have done so much for the "greater good."

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"They really use their platform in ways that is really unfathomable," Williams added. "I feel like they obviously have great respect from a lot of their peers, especially other athletes, people that really are looking for social change."

Took my niece to see @serenawilliams and @venuswilliams play tonight with @E_Reid35 at Arthur Ashe stadium. Lani lost it when Serena surprised her after the match!!!

Thank you so much Serena!!! pic.twitter.com/MpR2KOSU1V — Colin Kaepernick (@Kaepernick7) September 1, 2018

Kaepernick's kneeling protest in 2016 has since triggered a wave of protests to racial injustice and police brutality by other professional athletes.

Reid -- who is currently not signed to a team -- filed a grievance against the league in the spring and alleged he was colluded against by NFL owners due to his protests.

I know today will motivate Lani to be great! She couldn’t contain her excitement watching her favorite players @serenawilliams and @Venuseswilliams. Thanks Serena for surprising her after the match! pic.twitter.com/0gwoOlaoVI — Eric Reid (@E_Reid35) September 1, 2018

Kaepernick had also filed a grievance against the NFL, and will get the chance to take it to court.

An arbitrator denied the league’s request on Aug. 28 to toss his claim that he was colluded against because of his protests.

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