Spain's Rafael Nadal celebrates after winning his group stage match against Greece's Stefanos Tsitsipas during the match ATP Finals at The O2, London, November 19, 2020. Rafa Nadal kept alive his hopes of winning the year-ending ATP Finals for the first time as he beat Greek defending champion Stefanos Tsitsipas 6-4 4-6 6-2. Action Images via Reuters/Paul Childs

Spain's Rafael Nadal in action against Greece's Stefanos Tsitsipas during his group stage match ATP Finals at The O2, London, November 19, 2020. REUTERS/Toby Melville

Spain's Rafael Nadal in action against Greece's Stefanos Tsitsipas during his group stage match ATP Finals at The O2, London, November 19, 2020. REUTERS/Toby Melville

Spain's Rafael Nadal in action against Greece's Stefanos Tsitsipas during his group stage match ATP Finals at The O2, London, November 19, 2020. REUTERS/Toby Melville

Spain's Rafael Nadal in action against Greece's Stefanos Tsitsipas during his group stage match ATP Finals at The O2, London, November 19, 2020. Action Images via Reuters/Paul Childs

Spain's Rafael Nadal celebrates after winning against Greece's Stefanos Tsitsipas during his group stage match ATP Finals at The O2, London, November 19, 2020. Action Images via Reuters/Paul Childs