Facing Jelena Jankovic for the Premier Mandatory-level title, Williams seemed to be cruising to victory after taking the opening set. But at 62 11 she began grimacing in pain - seemingly the same sore back she talked about after her semifinal win - and Jankovic pounced, breaking for 2-1 in the second set.
But after the changeover the No.1-seeded Williams found her very best form once again, reeling off the next five games in a row to close the No.8-seeded Jankovic out in an hour and 28 minutes, 62 62.
Williams had more unforced errors than Jankovic in the match (29 to 24) but 20 more winners (29 to 9).
Williams now has 10 WTA titles this year, the most since 2007 (when Justine Henin also won 10). This was also her 73rd match win of the year, the most since 2003 (Kim Clijsters ended with 90, Henin 75).
Additionally, Williams now has 56 WTA titles to her name, separating herself from Virginia Wade and Lindsay Davenport for standalone seventh place on the all-time list of most WTA titles - she now trails only Martina Navratilova (167), Chris Evert (154), Steffi Graf (107), Margaret Court (92), Evonne Goolagong Cawley (68) and Billie Jean King (67). She had been tied with Wade and Davenport at 55.
Jankovic, the only player to play all 10 editions of the China Open from 2004 to 2013, is a popular face at the tournament - as runner-up she was asked to speak first during the post-match ceremony.
"I had a great week playing in this beautiful venue," Jankovic said to the crowd. "I always enjoy coming back here to the China Open - it's a pleasure for me to play in front of you all in the finals again.
"Serena was just too good today, and big congratulations to her."
The former World No.1 had more kind words. "I want to thank all of you - my Chinese fans, my Serbian fans and my friends who were supporting me. I appreciate it a lot and I hope to see you all next year!"
Williams then came up and took the microphone, carrying the 10th anniversary China Open trophy.
"I'm really excited - I didn't think I'd win this tournament starting out, so I'm really happy to be here holding the trophy," she said. "Congratulations to Jelena for such a great tournament as well.
"I really love tennis so much and it's fans like the ones here in China who are so exciting to come to. The fans make me want to play, they're so great here, so coming here is such a great opportunity."
After delighting the crowd with a few words in Chinese, the World No.1 had a few more thank yous. "Thank you very much to everyone out here. I love you in China and cannot wait to see you next year!"