The 20-year-old, who won the French Open at the start of June to reach the top of the rankings for the first time, again looked tense on Number One Court and her poor serving reflected her fragile state.
"I didn't play well," Ivanovic admitted afterwards. "But she plays very well on grass and I found it difficult adjusting to the timing of the ball.
"She has very powerful shot. She stays very low, so the balls are coming much faster through the air.
"And she was reading my serves very well. Even when I tried to kick it and bounce the ball high, she was still there on the ball."
"It was tough, I tried to hit my shots higher and get under the ball to play with speed but I was making too many mis-hits."
Zheng, who won the doubles title here in 2006 but missed the whole of the 2007 season because of an ankle injury, admitted she did not expect to win so easily.
"It was a surprise," Zheng said. "I believed I could win but I didn't think it would be in two sets, I though it would be tough and in three.
"When I got ahead, I just tried to keep going. I told myself she is a better player I had to be at the top of my game.
"She has a big serve and a big forehand but I tried to go for her backhand. She also served only so-so at first and it gave me a chance to play aggressively."
Zheng admitted she was surprised by her victory
Ivanovic started slowly and the first sign she was in trouble came as early as her second service game when a double fault saw her broken for the first time.
She had the opportunity to quickly break back but her game was already proving far too inconsistent to take advantage of the few opportunities Zheng was giving her.
And, soon after wasting the opportunity to reduce the deficit to 2-3, she quickly found herself 1-5 down after some big Zheng forehands ripped her serve to shreds.
Again, she had opportunities to reply but wasted four break points in the next game before Zheng wrapped up the set.
Ivanovic's struggles continued and she was broken again at the start of the second set with Zheng again dominant.
The Chinese player dropped her own serve immediately afterwards to give Ivanovic hope at 2-2.
But it soon became clear that another remarkable comeback from the 2007 semi-finalist was not on the cards as more double faults landed her back in trouble.
Zheng had wobbled before despatching Britain's Elena Baltacha in round two but showed no nerves on this occasion, serving out for a stunning victory.
I didn't have too much time to practice and to do all these little specific movements and things you need to in order to adjust to the grass
She will face Hungarian 15th seed Agnes Szavay on Monday for a place in the quarter-finals. Ivanovic's exit, together with that of third seed Maria Sharapova on Thursday evening, leaves the women's draw wide open.
She put her poor performance at the All England Club down to her failure to work hard enough on adjusting from clay to grass.
"After winning the French Open, I had a very emotional two weeks," Ivanovic explained.
"With my coaches, we cut down practice so I could recover and spend more time with myself. Next year I'll have to spend more time on court.
"I had to have some time off, so I didn't have too much time to practice and to do all these little specific movements and things you need to in order to adjust to the grass.
"From the first match on, I tried to find my game, but I felt like I was struggling a little bit.
"It took time to adjust to the grass because, especially after clay, it's a completely different game."
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