New York - Serena Williams reiterated her admiration for American football player turned activist Colin Kaepernick on Tuesday, and praised apparel maker Nike for featuring him in a new ad campaign.
WATCH: Angry Nike consumers burn products in protest
"Especially proud to be a part of the Nike family today. #justdoit," the 23-time Grand Slam champion tweeted before she headed out to book a US Open semi-final berth with a 6-4, 6-3 victory over Karolina Pliskova.
The ads prompted immediate calls for Nike boycotts over Kaepernick, who has been castigated by President Donald Trump and other conservatives for kneeling during the national anthem to protest racial and social injustice, a demonstration he began in 2016.
The former quarterback for the San Francisco 49ers, Kaepernick has been effectively blacklisted by the NFL, and Nike shares tumbled early Tuesday over worries a campaign around such a controversial figure could harm sales.
Williams told reporters after her victory over Pliskova that she thought the company made a "powerful" statement.
"He's done a lot for the African American community, and it's cost him a lot," Williams said. "It's sad. But he continues to do the best that he can.
"Having a huge company back him, you know, could be a controversial (decision) for this company, but they're not afraid," added Williams, who has had an endorsement deal with Nike since 2003.
"I feel like that was a really powerful statement to a lot of other companies."
As to whether athletes have a role to play in political and social discourse, Williams said she thought it was up to individuals.
"It's their choice," she said. "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. (I) just try to be the best that I can be, maybe I can just influence one person and that makes a change already."
US Open results on Tuesday, the ninth day of the 2018 tournament at Flushing Meadows (x denotes seeded player):
Juan Martin del Potro (ARG x3) bt John Isner (USA x11) 6-7 (5/7), 6-3, 7-6 (7/4), 6-2
Serena Williams (USA x17) bt Karolina Pliskova (CZE x8) 6-4, 6-3
Anastasija Sevastova (LAT x19) bt Sloane Stephens (USA x3) 6-2, 6-3