(CNN) -- Victoria Azarenka's alarming slump shows no sign of stopping after the World No.2 was beaten in straight sets by Jelena Jankovic at the WTA Championships in Istanbul Wednesday.
Azarenka, who won the Australian Open at the start of 2013, has now lost four of her past five matches including the U.S. Open final to Serena Williams.
The 24-year-old was beaten 6-4 6-3 as Jankovic produced one of her most impressive performances in recent years.
"It feels incredible to beat Azarenka here in Istanbul," she told the tournament's official website.
"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 had not beaten Azarenka for four years but after climbing back into the top eight for the first time in two years, she looked to be brimming with confidence.
Serbian star Jankovic will now face China's Li Na in the next round robin game.
Li defeated Italy's Sara Errani in straight sets and beat Jankovic in their previous meeting at the U.S. Open.
"Right now I'm just enjoying this win," Li said. "When I play Jelena it's always a tough match.
"We know each other very well. I played her at the U.S. Open, but that match doesn't mean anything tomorrow."
Top seed and World No.1 Serena Williams eased to a 6-2 6-4 victory over Poland's Agnieszka Radwanska
Wiliams has now won each of her eight meetings with Radwanska and is favorite to win the end of season event.
The U.S. star faces Petra Kvitova next in what promises to be an entertaining affair.
"Petra plays such a good game, she does everything well," Williams told reporters.
"She's just such a great player and a grand slam winner.
"It's definitely not an easy match for me but it's not an easy match for her either."
Meanwhile, in Switzerland, Roger Federer fought back to record a 4-6 6-3 6-2 win over Denis Istomin at the Swiss Indoors Wednesday.
The five-time champion recovered well after a slow start and remains on course for the end of season finals in London next month.
"I stared poorly, making too many errors, especially on the forehand," Federer told reporters.
"I was risking too much and it was not paying off.
"I had to cut that risk and was a bit lucky to win. I started fighting back and got better as the match went on.
"I'm happy that I was able to turn it around, I needed a match like this."