Serena: Nike's Kaepernick ad made 'powerful statement'

Serena Williams supports Nike's decision to make Colin Kaepernick the face of its "Just Do It" 30th-anniversary campaign.

Williams, who is endorsed by Nike, shared her thoughts on the campaign Tuesday after defeating Karolina Pliskova in straight sets to advance to the US Open semifinals.

"Having a huge company back him, could be a controversial reason for this company, but they're not afraid and I feel like that was a really powerful statement to a lot of, a lot of other companies," Williams said, according to Adam Reed of CNBC.com.

Williams also spoke about athletes' roles in social activism.

"I don't think athletes have a role to play," she said. "I feel like ... it's their choice."

Kaepernick and former San Francisco 49ers teammate Eric Reid were in attendance for Williams' victory over her sister, Venus Williams, in the third round of the US Open on Friday. Kaepernick tweeted a photo of Williams with his niece after the match.