Serena Williams saw Colin Kaepernick's new Nike ad campaign, and she was delighted to be a part of it.

Williams – who is preparing for the US Open quarter-finals – took time to praise Kaepernick and former San Francisco 49ers safety Eric Reid for their courage.

Kaepernick, who has been out of the NFL since he took a knee during the national anthem in 2016, was part of a new Nike ad campaign released on Monday.

"I think every athlete, every human, and definitely every African American should be completely grateful and honoured how Colin and Eric are doing so much more for the greater good, so to say," Williams said.

"They really use their platform in ways that is really unfathomable.

"I feel like they obviously have great respect from a lot of their peers, especially other athletes, people that really are looking for social change."

Williams takes on eighth seed Karolina Pliskova in the US Open quarters on Tuesday.