Going into the match Williams knew she was taking on one of her toughest match-ups - Jankovic is one of only two active players with four or more wins over her, alongside Venus Williams. But she hung tough early in the match, clawing her way back from 3-1 down to sneak out the first set, 6-4.
Things went awry from 1-all in the second set - the errors began piling up for Williams and Jankovic completely took over, reeling off five of the next six games to take the set, 6-2. The former No.1 was as sharp as ever in that second set, as well - she piled up 10 winners to just four unforced errors.
The No.1-seeded Williams was far from done, though. She went right to work from there, building a 5-1 lead in the third, and even though Jankovic kept inching her way back into it - she won the next three games and saved three match points - Williams had just enough to finish her off, making her way into net on her fourth match point and closing out the No.7-seeded Serb with a huge overhead, 64 26 64.
Though there were several twists and turns throughout the match, both Williams and Jankovic finished the day with even winners-to-unforced errors differentials - Williams was 40-40, Jankovic 22-22.
"I'm just really trying out here," Williams said straight after the match. "I think my body's a little tired after so many matches - I think yesterday I kind of hit a wall. But I'm just fighting out here.
"Honestly I'm just happy to still be in the tournament, and I'm honored to be in the final again."
This will be Williams' sixth time in the final of the TEB BNP Paribas WTA Championships. She has won the title three times, in 2001, 2009 and 2012, and was a runner-up twice, in 2002 (to Kim Clijsters) and 2004 (to Maria Sharapova). She is after her fourth title here - only three players have won four or more titles at this event (Martina Navratilova won it eight times, Steffi Graf five and Chris Evert four).
Awaiting Williams in the final is No.4 seed Li, who won her semifinal earlier on in the day (read more here). Williams leads Li in their head-to-head series, 9-1, including three straight set wins this year.
This is the first time in the history of the tournament that both finalists are 30-somethings.