The top-ranked Djokovic has won the tournament five times — one short of Roger Federer‘s record.
Schwartzman broke Djokovic for a 2-1 lead in the first set, but the 28-year-old Argentine immediately gave it back as he struggled with his first serve.
At 3-3, Djokovic won five straight games to close out first set and build 2-0 lead in second.
Djokovic hasn’t lost an opening ATP Finals match since 2007 against David Ferrer. The 33-year-old Serb has never lost to Schwartzman in six matches.
The Argentine player — without a title this year and making his debut at the ATP Finals — reached the French Open semifinals and the Cologne Championship final. He also made it to the Italian Open final, losing to Djokovic.
The ATP Finals move to Turin, Italy, next year, after 12 years in London.