Serena Williams praised Nike for creating a "powerful statement" by making Colin Kaepernick the face of an advertising campaign.
The quarterback appears alongside the message: "Believe in something. Even if it means sacrificing everything", as part of the 30th anniversary of Nike's Just Do It campaign.
Kaepernick was the first NFL player to kneel at games during the national anthem as a protest against social and racial injustice.
He inspired many other players to join in, earning the ire of people including Donald Trump, and the protest has cost Kaepernick his NFL contract.
Williams, who is sponsored by Nike, has been a supporter of Kaepernick and he was at Flushing Meadows on Friday to see her defeat sister Venus in the third round.
Following a quarter-final victory over Karolina Pliskova, Williams said: "He's done a lot for the African-American community, and it's cost him a lot. It's sad. But he continues to do the best that he can to support.
Believe in something, even if it means sacrificing everything. #JustDoIt pic.twitter.com/SRWkMIDdaO
— Colin Kaepernick (@Kaepernick7) September 3, 2018
"Having a huge company back him could be a controversial reason for this company, but they're not afraid. I feel like that was a really powerful statement to a lot of other companies."
Williams declined to answer whether she would take part in similar protests in tennis given the chance, saying: "I think everyone has a choice to do what they choose to do.
"Whether they protest it, which is a peaceful protest, or not, it's the choice of being American. It doesn't make them less American.
"And I think that's also something that's really interesting, is the fact that we all make up this world, because we have different views on different things, but that doesn't mean we can't be loving toward each other."
John Isner, a supporter of Trump who has previously been very critical of Kaepernick, refused to answer a question about the matter following his defeat by Juan Martin del Potro.
"I don't see how that has any relevance here, so I really don't appreciate that question," he said.
