Serena Williams has backed the "really powerful statement" made by Nike following the sportswear giant's high-profile link-up with Colin Kaepernick.

Kaepernick was unveiled on Monday as one of the faces of a new advertising campaign from Nike, the NFL's official apparel suppliers.

The 30-year-old has not played in the NFL since opting out of his contract with the San Francisco 49ers in 2017, following a season in which he was both praised and criticised for kneeling during the national anthem in protest at police brutality and perceived racial injustice in the United States.

Unsurprisingly, given the mixed reaction to Kaepernick's protest, Nike's decision has prompted strong views of support from certain quarters and condemnation from others.

Tennis great Williams – a long-term Nike athlete – tweeted on Monday that she was "especially proud to be a part of the Nike family today".

Following her 6-4 6-3 victory over Karolina Pliskova in the US Open quarter-finals on Tuesday, she was asked to comment on Kaepernick's deal with the company.

"Well, I mean, 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," said former world number one Williams.

"Having a huge company back him, you know, 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 was also questioned about the role of athletes in social activism and responded: "I don't think athletes have a role to play. I feel like they can choose or they cannot choose. And it's their choice. I think that's one great thing God gave us, is the freedom of choice."

"My choice is to choose to just be the best, try to be, I'm not always perfect. Actually I'm never perfect. Just try to be the best that I can be. Maybe I can just influence one person, and that makes a change already."

"I think also something that's really interesting, is the fact that we all make up this world, because we have different views and different views on different things, but that doesn't mean we can't be loving toward each other."

The NFL released a statement on Tuesday saying the issues raised by Kaepernick "deserve our attention and action".

Last November Kaepernick filed a grievance against the NFL in which he alleged that franchise owners colluded to keep him from signing with another team. An NFL request to dismiss the case was denied last month, meaning there is sufficient evidence for it to go to trial.