Wearing Asics sneakers, a Nike top and $400 Monreal London tracksuit pants, the royal mom, 34, took part in a tennis workshop at a high school gym in Edinburgh, Scotland.
And even though she was dressed down for the event, Kate pulled back her hair to show off the stunning gold-and-diamond leaf earrings from Kiki McDonough she had been wearing earlier in the day.
Led by tennis coach Judy Murray – mom of British tennis pro Andy Murray – Kate took part in a Tennis on the Road session, which "aims to grow the game from the grassroots up,” a spokesperson for Judy explains.
Kate sat on the gym floor with a group of parents, teachers, coaches and young leaders as they listened to Murray.
Many of the drills Murray demonstrated were dreamed up to help her own sons, Andy and Jamie, when they were kids.
Kate eagerly joined in on the fun and was joined by a little girl for an exercise with blue balloons that aimed to aid hand-eye coordination.
Want to keep up with the latest royals coverage? Click here to subscribe to the Royals Newsletter.
The event marked the royal's third appearance of the day. Earlier, she visited a facility run by her charity, The Art Room, and checked in on the work of one of the mental health charities she works with – Place2Be.