Li had showed her battling qualities in her comeback win over Williams' sister Venus yesterday and fended off four match points to frustrate the American's charge.

But Williams stamped her class on the contest when it counted most in each tie-beak to claim her 50th win at Melbourne Park.

The 28-year-old served out the match with an ace and let out a loud scream of delight after the closely-fought match.

"I had so many match points and I blew it," a relieved Williams said.

"I knew on serve (in the tie-break) I had the chance to close it out and I did. I had to because she is an amazing fighter."

Williams will now aim for a 12th Grand Slam title against the winner of today's other semi-final between returning Belgian Justine Henin and Zheng Jie.

Should Williams achieve that she will equal the mark of Billy-Jean King, who will be in Melbourne for the final.

"That's the goal," Williams said. "I have a long career to keep trying to do it, but if it doesn't happen this year then I'll keep trying."

Williams broke in the opening game of the match and offered little sign she would surrender the advantage as she cranked up her serve.

The American did not lose a point on first serve in her opening four service games, although Li characteristically clung in the set.

The 27-year-old Chinese, who will move into the top 10 for the first time after the tournament, proved her battling qualities as she fended off a set point on her serve and then, with Williams serving for the set, found a way to break back and level at 5-5.

The set went to a tie-break where Williams prevailed with a second-serve ace that clipped the line.

Williams had never lost a match at Melbourne Park but struggled to overcome Li who saved three match points on her serve at 5-4 and then another in her next service game to take the match to a second tie-beak.

Li's resistance was finally broken, however, as Williams stormed clear in the tie-break before wining it with an ace.