Zheng, who just turned 20 years old this year, had shown promise in the past - she's done very well in doubles and in WTA 125K Series and ITF Women's Circuit events, and she's also pushed top players to three sets a few times. But she had never beaten anyone in the Top 60 going into this match.
With some rock solid baseline play and deft touch at the net she changed all that, outfoxing the No.14-ranked, No.1-seeded Jankovic for the better part of an hour and 27 minutes, her most dominant stretch being winning nine of 10 games from 3-2 down in the first set to build a massive 6-3, 5-1 lead.
Jankovic staged a late rally, winning two games to close to 6-3, 5-3 and even holding break point in the next game to get it back on serve - but Zheng held her nerve and her serve to end it, 6-3, 6-3.
"Jankovic is by far the highest-ranking player I've ever defeated," Zheng said.
"Being able to defeat a top player like her definitely gives me more confidence in future matches."
Zheng's first Top 20 win also brought her the first WTA quarterfinal of her career. And all just a day after being named one of the four players going to the WTA Rising Stars Invitational in Singapore.
The only other match produced the opposite of an upset. Peng Shuai, now the highest seed left at the tournament at No.2, completely dominated Russian qualifier Elizaveta Kulichkova, 6-1, 6-2, reeling off six games from 1-0 down in the first set and another six games from 2-0 down in the second set.
The battle for the last spot in the doubles field at the BNP Paribas WTA Finals Singapore presented by SC Global also heated up, as No.1 seeds Martina Hingis and Flavia Pennetta fell to Sorana Cirstea and Andreja Klepac, 3-6, 7-6(1), 10-6. They led by a set and three times by a break in the second set.
Now if No.2 seeds Alla Kudryavtseva and Anastasia Rodionova win the title this week, they will become the eighth and final team in the WTA Finals doubles field. And if they don't win the title this week, the battle continues on to Moscow next week - if Kudryavtseva and Rodionova reach the final there or Hingis and Pennetta don't win the title, Kudryavtseva and Rodionova are in Singapore.