This tournament may one day be remembered as a watershed moment for Asian tennis with two Chinese making the semi-finals for the first time, but a potential new world order was left on hold a little longer.

Williams used all her power and big-match experience to beat Li Na 7-6 7-6 in an absorbing centre court contest to remain on course to defend the title she won for the fourth time last year.

The American was made to work much harder than she expected but the top seed was able to raise her game when she needed to.

"I am happy I was able to pull it out, it was really close," Williams said.

"I wasn't at my best today but I'm still here -- which is shocking -- and I'm just going to do whatever I can to stay."

Henin, in only her second tournament since coming out of retirement, demolished Zheng Jie 6-1 6-0 to continue her fairytale comeback to the sport she once dominated.

The Belgian, getting better with every match she plays, was at her ruthless best against Zheng, wrapping up a horribly one-sided victory in 51 minutes.

Few people gave Henin any real hope of winning the tournament when she arrived in Melbourne two weeks ago but the 27-year-old is starting to believe in herself.

"The dream continues," Henin said. "I'm going to play the number one in the world. She's a fighter as she's proved here, and I'll do my best and try and get the title."