Stephanos Tsitsipas of Greece overcame Austrian Dominique Them 6-7 (6-8), 6-2, 7-6 (7-4) on Sunday to win the WTA season final.
The 21-year-old Greek star ended the match in London in two hours 26 minutes.
Tsitsipas became the youngest player to win the tournament since 2001, when he won the 20-year-old Australian Lleyton Hewitt title.
It is the fourth consecutive edition of the tournament, with Andy Murray winning the title in 2016, Gregor Dimitrov in 2017 and Alexander Zverev in 2018.
Boris Becker won the title for the first time in his career in 1988, Stefan Edberg in 1989, Andre Agassi in 1990 and Pete Sampras in 1991.
Tsitsipas received a cash prize of $ 2,656,000, in addition to 1,300 points.
Tsitsipas has become the first player to reverse his late lead in the first set and win the title since David Nalbandian did it in 2005.