*Serena has to fight against Zheng
*Dane Wozniacki stuns Dementieva
(adds late match)
By Simon Evans
MIAMI, March 30 (Reuters) - World number one Serena Williams struggled to a 7-5 5-7 6-3 win over China's Zheng Jie on Monday to earn a place in the quarter-finals of the Sony Ericsson Open.
After Sunday's surprises, another contender fell with Russian fourth seed Elena Dementieva making a surprise exit at the hands of Danish teenager Caroline Wozniacki 7-5 6-4.
Serena's sister Venus came from a set down to beat Poland's Agnieszka Radwanska 4-6 6-1 6-4, keeping alive the chances of an all-Williams semi-final.
It all started as business as usual for Serena, who broke Zheng's first two serves to take a 5-0 lead in the first set.
But then the powerfully-built American lost control -- and her cool -- as she gave up two serves and ended up level at 5-5, twice throwing her racket to the ground in frustration.
In the end her powerful serve saw her through the set but if the American thought that she had broken the will of the 25-year-old 17th seeded Zheng, she was sorely mistaken.
The first four games of the second set were all broken and it was level at 5-5 when Zheng, showing smart backhand work, produced another break and then held serve to even up the match at one set all.
Williams finally found some form in the decider and finished off in style with three aces to win the final game to love and claim her place in the last eight.
"I don't think I played great and I thought she played probably her best tennis yet," said Williams.
"I think I started out great and then maybe lost my focus Next thing I know, I was 5-5 in the first set, which should have never been. I am just happy to get through it," she added.
Serena will face her third consecutive Chinese opponent in the tournament after Li Na beat Ekaterina Makarova 6-7 6-2 6-2 in her fourth round match.
The 18-year-old Wozniacki came back from a break down in each set to reach her sixth quarter-final of the season.
The result continued a poor tournament for the top East European women with Russians Dinara Safina and Vera Zvonareva and Serbians Jelena Jankovic and Ana Ivanovic, all top-10 players, failing to make the last 16.
Wozniacki, named the WTA's newcomer of the year for 2008, is enjoying a breakthrough season and has climbed to 12th in the rankings.
"I'm just enjoying every day and every match that I'm playing," said the Dane, whose parents are Polish.
"That I keep winning is just great. I just felt like I was playing really well today. I just got everything together," she told reporters.
Wozniacki will play Svetlana Kuznetsova, who comfortably won her all-Russian fourth round tie with Alisa Kleybanova 6-2 6-1.
Hungarian Agnes Szavay, who eliminated Ivanovic, lost 6-2 6-4 to Belarussia's Victoria Azarenka.
Azarenka's quarter-final opponent will be Australian Samantha Stosur whose impressive form continued with a 6-4 6-4 win over France's former world number one Amelie Mauresmo.
(Editing by Nick Mulvenney.
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