Sitting alongside her husband, Prince William, her brother-in-law Prince Harry and Prince Edward, Kate cheered on English boxer Scott Fitzgerald and Azumah Mohammed of Ghana before heading over to the hockey match.
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The audience burst into applause when they saw that the royals had arrived.
Kate, who had previously been at the gymnastics final, looked cool and casual as she slipped out of her blazer to show off a sleeveless striped shirt.
An avid hockey fan, sporty Kate was hockey captain of her school team, and in 2012 she took part in a training session at the Olympic hockey arena with Team GB athletes.
"The last time I played was 2005, maybe. It feels like I haven't held a stick for ten years", she revealed at the time, before adding: "This is going to be so embarrassing. My brain thinks I can do all these wonderful things but my body just doesn't play ball."
The royal visitors – who are all avid sports fans – will also attend the Games on Tuesday, and other events they will have a choice of watching include athletics, badminton, hockey, lawn bowls, netball, shooting, squash, swimming, table tennis and weightlifting.
William and Kate left their just-turned one-year-old son Prince George at home in their Kensington Palace apartment, although they were both spotted on their phones throughout the day, possibly checking in with George's nanny.