The Dane lost to Serena Williams in the US Open semi-finals, meaning that she is still without a Grand Slam, but controversially, that has not prevented her from topping the rankings.
'It's very tough to be number one. But it's even tougher to stay there because everyone wants to beat you,' said the 21-year-old, as the first round of the $2.05 million tournament got under way.
'It's always nice to be back to a tournament where you are defending the title. It's a special feeling. I am going to try to do my best to defend it.'
Crowd favourite Maria Sharapova, who won the title two years ago and was also champion in 2005, returns as the second seed after a 'rewarding' season that has seen her achieve her highest ranking since she returned from shoulder surgery in 2009.
It is a sharp contrast from last year, when the Russian exited the Tokyo event in a first-round upset to Japanese veteran Kimiko Date Krumm.
'I had a tremendous year, back in number two in the world. So I am quite excited about the future as well,' said Sharapova, a Wimbledon finalist and semi-finalist at the French Open this year.
Her early elimination in the US Open earlier this month was a disappointment, but 'in terms of a bigger picture, I am a lot happier' than last year, she said.
'I have been very healthy. My shoulders have been feeling really good. This time of the year, it's so important to keep that going,' she said as the season nears its end.
The top two women will face a strong field of rivals including Wimbledon winner Petra Kvitova, US Open winner Samantha Stosur, world number three Victoria Azarenka and number four Vera Zvonareva.