Victoria Azarenka couldn't summon her best game to stop Jelena Jankovic at the WTA Championships. (Matthew Stockman/Getty Images)

ISTANBUL -- Jelena Jankovic defeated Victoria Azarenka 6-4, 6-3 on Wednesday in round-robin play at the WTA Championships. Azarenka admitted afteward that she's mentally exhausted after trying to maintain a consistent level over the last two years.

"I think it's pretty obvious," a visibly drained Azarenka said when asked about any mental fatigue.

There is little at stake for Azarenka this week. She has no chance of overtaking Serena Williams for the No. 1 ranking, and her No. 2 ranking is secure regardless of what any other player does.

"It's just mentally tough right now," she said. "Just tough to get things started a little bit."

Since making the final of the U.S. Open in September, Azarenka has won just one match in three tournaments -- an unconvincing 7-6 (4), 6-2 victory over Sara Errani on Tuesday. Azarenka chalked up the subpar performance against Errani to taking a few weeks off after an unsuccessful Asian swing, but she ran up against an in-form Jankovic, who is playing some of her best tennis in more than two years.

"I cannot sit and say that I played well," Azarenka said. "Of course she was better today, but I let a lot of balls go."

The 28-year-old Serb kept Azarenka under pressure throughout the match. She earned 17 break points against Azarenka, who has struggled with her serve since injuring her right knee at Wimbledon. While Azarenka tried to blast her way past Jankovic -- she hit 30 winners -- she also gave away too many points, hitting 40 unforced errors.

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Jankovic was the steadier and craftier player, mixing in consistent hitting from the baseline with scrambling defense punctuated by perfectly weighted lob winners and passing shots. She hit 14 winners to just 13 unforced errors in the one-hour, 49-minute match.

"I think the difference today was that I was serving much better," Jankovic said. "I was able to hold my serve and stay focused when I was serving and I waited for my chances to break, and she made some double faults at times and I took advantage of that. So I think that for my first match, playing Vika was not easy, and I'm really happy I was able to win. Gave me a lot of confidence."

"It's been a long year," Azarenka said. "It's been a tough year. It's been tough two years, so that consistency I have been playing with, it's sometimes difficult to keep all the time."

Though Azarenka has played the fewest tournaments (14) and fewest number of matches (51) of the eight women in the field, injuries and illness have been a strain on her year.

"It's tough at the end of the year to play against the best players," she said, "because every match you have to go and dig deep, and sometimes your motivation is not there enough to know the capacity how deep can you dig."

Serena Williams, who beat Agnieszka Radwanska 6-2, 6-4 to move to 2-0 in round-robin play, understood Azarenka's perspective.

"I think it's natural, normal," she said. "I'm always feeling like that toward the end [of the year]."

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Though she added that she's "barely hanging on," Serena's hunger and motivation aren't an issue.

"I'm definitely motivated," she said. "I don't want to come all this way to not do well."