The 35-year-old supermodel challenged her friend to a cocktail making and drinking competition at the customised bar of her London home, and was delighted to come out victorious.
A source said: "Kate and Lily each made up various cocktails and had a competition to see how many they could drink."
Kate was crowned the winner at around 4am last Thursday and is now looking forward to receiving a mystery prize from the 23-year-old 'Fear' singer.
The impromptu contest took place after Lily's concert in London, and the singer planned to take a portrait of the model by graffiti artist Banksy from Kate's lounge if she won.
The source added: "Kate doesn't know yet what Lily will be giving her. They both became pretty drunk by the end."
Lily and Kate became friends in January after the model came to the 'Smile' star's concert at London's Roundhouse venue.
They then enjoyed a shopping spree together before spending time at Paris Fashion Week.
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