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 (above).

"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.