The Oscar-winner and her husband Sam Mendes joined a packed crowd in cheering him on to the final four on Wimbledon's Centre Court this week.
Springing to her feet after the big points and applauding enthusiastically, The Reader actress must have given him a boost.
The Scotsman sailed through to the semi-finals, destroying former World No 1 Juan Carlos Ferrero in under two hours.
Now two main obstacles stand between the 22-year-old and a historic victory – no British man has won the singles title at the All England Club since 1936 - his friend, American player Andy Roddick and formidable five–time champion Roger Federer.
The 'Kate' effect was also felt closer to her home town of Reading a few days ago when she thrilled pupils at her old stage school with a surprise visit.
Kate, patron of Berkshire-based amateur dramatic group Starmaker, answered questions from budding actors and actresses before participating in a drama workshop.
Students had no idea about the VIP visit until the curtain drew back to reveal the Hollywood leading lady waving and blowing kisses.
To make sure they weren't too star-struck to ask questions, the 33-year-old told them: "Don't be shy, because I am very normal".