Though she had some major struggles on break points - she converted just four of 17 - the No.7-seeded Jankovic was just too solid for the No.2-seeded Azarenka throughout the match. She fired 14 winners and gave away only 13 unforced errors, a +1 differential - meanwhile, Azarenka had a -10 differential of 30 winners to 40 unforced errors. At the end of the day Jankovic moved on, 64 63.
After a few hellos to her family back home into the camera, Jankovic was interviewed on court.
"It feels incredible to beat Azarenka here in Istanbul," she said. "First of all I was so excited to qualify here and give myself a chance to play against the best players in the world once again, and to win today is amazing. Every match in the round robin format counts, and every match is difficult too.
"I'm really happy. Beating Vika will give me a lot of confidence going into the next matches."
Jankovic, a former No.1, had lost her last 17 matches against Top 4 players - the last time she had beaten anyone in the Top 2 was at Rome in 2010 (where she beat a No.1-ranked Serena Williams).
The Serb's win over the No.2-ranked Azarenka was also her fifth career win over a Top 2 player. The first four came at Birmingham in 2007 (No.2 Maria Sharapova), Cincinnati in 2009 (No.1 Dinara Safina), the WTA Championships in Doha in 2009 (again No.1 Safina, though it was via retirement after just two games) and the aforementioned win at Rome in 2010 (No.1 Williams, saving match point).
Azarenka has now lost four of her last five matches, starting with the US Open final (to Williams), then opening round losses at Tokyo (to Venus Williams) and Beijing (to Andrea Petkovic), and she is now 1-1 this week (beating Sara Errani in her first round robin match, then falling to Jankovic today).
Azarenka will have a day off on Thursday - Jankovic will play her second round robin match against No.4 seed Li Na (Li leads the head-to-head series, 5-4 - read more about Li's win on Wednesday here).