The 26-year-old Serb, a six-time grand slam winner, won a compelling match 6-1, 3-6, 7-6 (7-3).
Djokovic has now won 20 straight matches in China. Both he and Del Potro will compete in the Barclays ATP World Tour Finals in London at the start of next month, along with world No1 Rafael Nadal and world No7 Roger Federer.
Britain’s Andy Murray has withdrawn due to injury.
“All the way up to the last point, I didn’t know if I was going to win the match,” Djokovic said yesterday. “It was a very even match. My motivation is there always, especially when I’m playing finals of such a big event.”