The 30-year-old American Football player has not been selected by an NFL team for the last two years after starting the ‘Take a Knee’ protest against social and racial injustice, which led to a number of black NFL players joining him in kneeling down during the national anthem.
Kaepernick’s face appears in the advert alongside the words: “Believe in something. Even if it means sacrificing everything.”
The campaign was launched on Monday night at part of Nike’s 30th anniversary of the ‘Just Do It’ programme, but it has caused huge debate in the United States as supporters and critics of Kaepernick’s protest voiced their opinions.
Williams, who is also sponsored by the American sportswear giant, joined the support for the campaign though, and having herself been a vocal supporter of equality during her career – when it comes to gender and race – she said that it’s “sad” what Kaepernick has had to sacrifice in order to take a stand.
"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,” Williams said on Tuesday after her victory over Karolina Pliskova in the US Open quarter-finals.
"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."
Kaepernick’s protests have not gone without criticism, with Donald Trump regularly speaking out against the NFL star and questioning his loyalty to the United States, having previously claimed that he is disgracing the memory of US veterans by not standing for the national anthem.
Williams refused to say if she would feature in a similar campaign, but stressed that Kaepernick’s decisions to protest in the way that he has does not affect where he comes from or his loyalty to his country.
"I think everyone has a choice to do what they choose to do,” the 23-time Grand Slam winner added.
"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."
On the other end of the scale, American John Isner refused to answer questions about Kaepernick’s advert, having previously criticised him and taken a supporting stance of President Trump.
"I don't see how that has any relevance here, so I really don't appreciate that question," he said following his US Open defeat by Juan Martin del Potro.
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