Ms Miller teamed her high-neck frock with chunky ankle boots, a black long-strap handbag, scraped back ponytail and bold lip.
Sienna was joined by sister Savannah and mum Jo Miller at the event, held at the Ralph Lauren store on Brompton Road.
Ralph Lauren have this year continued their collaboration as Wimbledon's official outfitters to celebrate the 125th anniversary of the tennis tournament.
What do you think of Sienna's tennis whites? Let us know in a comment below.