Victoria Azarenka of Belarus reacts after losing a point against Andrea Petkovic of Germany during their first round match of the 2013 China Open tennis tournament in Beijing, Sept 30, 2013. [Photo/Icpress.cn]

BEIJING - Defending champion and world No 2 Victoria Azarenka of Belarus became the biggest seed casualty at the China Open as she was upset by German Andrea Petkovic at the opening round on Monday.

The Belarussian was troubled with too many unforced errors and lost the match 6-4, 2-6, 6-4. With the victory, Petkovic, a former top 10 player, improved her win-loss record against Azarenka to 1-2.

The match had several momentum swings - first Petkovic went up a set and a break at 6-4, 2-1, then Azarenka battled back to take the second set and go up 2-0 in the third. Then the two found themselves locked at 4-all in the third.

Petkovic lifted her game just in time to take the last two games at love - a love break, then a love hold - to end it after two hours and 22 minutes.

"I'm very happy with my performance," Petkovic said. "I was happy with my movement and my serve for the most part, and I managed to get a lot of balls back very deep and very strong. Parts of the match were a really, really good level, some parts I lost a bit of energy, but overall I'm very happy."

Petkovic always seems to lift her game in Beijing - she reached the biggest final of her career, in fact the only Premier-level final of her career, right here in 2011.

"I always play really well here," Petkovic said. "I lost in the first round here last year but I was playing Jelena Jankovic, who was playing really well that day. I really like the courts. I don't know, it's just a tournament that goes with my play somehow! "

Andrea Petkovic of Germany returns a shot against Victoria Azarenka of Belarus during their first round Womens Singles match of the 2013 China Open tennis tournament in Beijing, Sept 30, 2013. [Photo/Icpress.cn]

Next up for Petkovic is another former China Open champion, the only two-time champion in the tournament's 10-year history, Svetlana Kuznetsova. Kuznetsova beat Hsieh Su-Wei of Chinese Taipei earlier on at 6-1, 6-1.

Kuznetsova and Petkovic are tied in their head-to-head series at 2-2.

Defending champion Azarenka wasn't happy with her performance at all. "It was an awful match and a very bad performance from me, so there's not much to say about it," the Belarussian said.

Azarenka also lost her first match at last week's Toray Pan Pacific Open in Tokyo, to Venus Williams.

"If I did it all again, I probably should have taken a longer break and prepared myself better. I don't feel like I was ready to play, the preparation wasn't enough, I cannot deny that. It's my mistake for not paying enough attention after the US Open how I managed my time and managed my health.

"But you cannot put yourself down too much. In the end the important thing is how you come out of it. I'm going to go back, reevaluate and practice more. I just needed more preparation, and that's what I'm going to get now before the last tournament. I'll just look at the positives. I have three weeks now," the second seed said.