The No.5-seeded Li, who had lost the pair's last three meetings, came out blazing against the No.9-seeded Jankovic, storming out to a quick 3-0 lead and, after dropping two games in a row, extending that to a 5-2 lead. She missed out on set point in that game, though, and lost her serve for 5-3.
But that's when the floodgates opened - Li reeled off 15 points in a row from there, breaking back at love to take the first set then storming ahead 2-0, 40-0 in the second set, and although Jankovic stopped the points streak she would never stop the games streak, as Li didn't drop another game.
Li's final numbers were 20 winners to 12 unforced errors - she was an even more devastating 10 and 2 in the second set (the exact numbers Serena Williams put together in her second set earlier on).
"It's very tough to play against Jankovic because she was a former No.1 and has very good defense," Li said after the match. "I still remember the last time we played in Rome. I was feeling pretty good and I was playing very well, but I still lost the match. I was pretty sad after that match. So before this match, Carlos was just telling me to remember how I felt after that match, and I was like, 'Okay, yeah.'
"I was really, really happy about the way I was playing today. I was trying to serve and volley and coming into the net so that I could win the points fast, because she's so good in the rallies."
How did Li feel about playing the night session on Arthur Ashe Stadium, her first time in years?
"When you play first match of the day, you have the whole day to do everything else," Li said. "But that's not like the night session. For the night session you have to wait the whole day and play in the night. But after I won the match tonight I was feeling pretty okay, just that it was a little bit late."
Li is now through to the quarterfinals of the US Open for the second time, having gone this far in 2009 (falling to Kim Clijsters). The Chinese is through to her ninth overall Grand Slam quarterfinal, and is 4-4 in her previous eight (going further three times in Melbourne and - very memorably - once in Paris).
Awaiting Li in the quarterfinals is No.24 seed Ekaterina Makarova. Li has beaten Makarova in all three of their previous meetings, including a three set win in the third round right here five years ago.
"Another lefty, but more flat," said Li, who played Laura Robson in the third round. "But for me today I don't have to think about the next round. I just want a good rest and I'll prepare for that tomorrow."