LONDON, ENGLAND - NOVEMBER 08: Roger Federer of Switzerland serves in a practice session during previews for the Nitto ATP World Tour Finals at The O2 Arena on November 08, 2019 in London, England. (Photo by Alex Pantling/Getty Images)
LONDON, ENGLAND - NOVEMBER 08: Roger Federer of Switzerland serves in a practice session during previews for the Nitto ATP World Tour Finals at The O2 Arena on November 08, 2019 in London, England. (Photo by Alex Pantling/Getty Images)