William was joined by his wife Kate, who smiled and cheered while watching from the sidelines, wearing a white and sky blue patterned dress reportedly designed by Beulah London.
The prince played in defence, chasing the ball with a sturdy backhand swing and scoring two goals in an intense match.
For his second strike, the father-of-three hit the ball with such force he broke his mallet.
William played in a number four shirt on the winning US Polo Assn team, alongside Aiyawatt “Top” Srivaddhanaprabha, the chairman of Leicester City FC.
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With Windsor Castle overlooking the Guards Polo Club, the prince faced off against other polo teams, BP Healthcare and Standing Rock, in the round-robin tournament.
The charity cup is raising funds and awareness for 11 charities supported by William and Kate.