The royal sisters-in-law watched Simona Halep defeat Serena Willams in the women's singles final.
The 5-year-old royal also has another connection to Serena Williams.
Middleton and husband James Matthews welcomed their son in October.
Middleton and her husband, James Matthews, welcomed their new baby son.
Middleton will likely ditch her Duchess of Cambridge title for something more sentimental: the Princess of Wales.
The prince, born April 23, is the youngest child of Prince William and the former Kate Middleton.
Duchess of Cambridge's sis is writing a fitness column during her pregnancy.
Our first glimpse of the former Kate Middleton since she gave birth!
Celebrities attending the royal wedding of Prince Harry and Meghan Markle included Oprah Winfrey, Serena Williams, Elton John, Idris Elba, George and Amal Clooney, Victoria and David Beckham, Priyanka Chopra, and more.
David Matthews was detained and questioned in France.