Spain's Rafael Nadal reacts during his group stage match against Germany's Alexander Zverev during the match ATP Finals at The O2, London, November 11, 2019. World number one Rafael Nadal made a disappointing start at the ATP Finals as he suffered a first career loss to Germany's reigning champion Alexander Zverev, going down 6-2 6-4. Action Images via Reuters/Tony O'Brien

Germany's Alexander Zverev in action during his group stage match against Spain's Rafael Nadal during the match ATP Finals at The O2, London, November 11, 2019. The Spaniard, seeking a first title at the event and hoping to secure the year-end number one ranking for a fifth time, showed no sign of the abdominal injury sustained in Paris. Action Images via Reuters/Tony O'Brien

Spain's Rafael Nadal in action during his group stage match against Germany's Alexander Zverev during the match ATP Finals at The O2, London, November 11, 2019. Action Images via Reuters/Tony O'Brien

Germany's Alexander Zverev celebrates after winning his group stage match against Spain's Rafael Nadal during the match ATP Finals at The O2, London, November 11, 2019. Action Images via Reuters/Tony O'Brien

Germany's Alexander Zverev with Spain's Rafael Nadal after winning their group stage match ATP Finals at The O2, London, November 11, 2019. REUTERS/Toby Melville

Spain's Rafael Nadal waves after losing his group stage match against Germany's Alexander Zverev during the match ATP Finals at The O2, London, November 11, 2019. REUTERS/Toby Melville