Caroline Wozniacki of Denmark returns the ball against Lucie Hradecka of the Czech Republic during their women's singles match in the China Open tennis tournament at the National Tennis Center in Beijing October 3, 2011.[Photo/Agencies]

BEIJING - World number one Caroline Wozniacki saw off a tenacious challenge from Lucie Hradecka to open the defence of her China Open title with a 3-6 6-0 7-5 first round victory on Monday.

The top seeded Dane struggled to find her rhythm against the powerful Czech's serve in the first set but asserted her superior ranking in the second, only to be forced into fight for survival in a nail-biting decider against the world number 49.

"It wasn't a pretty match, but I'm through," the relieved Dane told reporters.

"She's a player that doesn't give you any rhythm. She's serving at 200 kilometres per hour, so when it's on, it's difficult.

"But in the second set, her first serve percentage dropped, and that made it easier for me to start the rallies," she added.

The 21-year-old, who was crowned world number one at this event a year ago but has struggled for form in recent tournaments, said maintaining her top ranking was paramount.

"If you've been number one pretty much the whole year, you want to finish there as well. It would be a little bit bitter to lose it in the last week of the season," she added.

"So definitely, I just give it my all in the last few tournaments."

Outgunned in the first set, the tournament was in danger of suffering another upset before Wozniacki rallied to turn the match around against her 26-year-old opponent.

Local favourite Li Na's shock first round loss on Sunday and the withdrawal of several top stars, including world number two Maria Sharapova, had put a damper on the competition, but organisers will be relieved to see Wozniacki still in the draw.

In another result, Austria's Tamira Paszek became the first woman to reach the third round when she rallied past Petra Cetkovska of the Czech Republic 1-6 6-4 6-2.

In the men's draw, Germany's Florian Mayer beat Albert Ramos 6-2 6-4 and Croat Marin Cilic trounced China's Wu Di 6-2 6-0.

Defending men's champion and world number one Novak Djokovic pulled out two days before the start of the tournament because of a back injury.