The 21-year-old Brisbane resident beat world no.15 Jankovic 7-6 6-0 in the first round on Sunday.
The hard-hitting Tomljanovic beat the former French Open finalist in 105 minutes after a tight first set.
World No.62 Tomljanovic, who was well supported by the crowd, is expected to gain Australian citizenship in the next few months.
Tomljanovic needed to hold her nerve in the first set against Jankovic, breaking straight back after dropping her serve in just the third game.
Jankovic broke again in the ninth game but Tomljanovic dug deep to force a tiebreak.
The crowd favourite then saved two set points and won her first to take the tiebreaker 8-6.
Tomljanovic then showed no mercy in the second set, winning it in 35 minute.
In another upset, former champion Kaia Kanepi of Estonia downed fifth seed Andrea Petkovic of Germany in three tense games 6-4, 5-7, 6-4.