Serena Williams saved two match points to beat fourth seed Kim Clijsters in a thrilling Australian Open semi-final.

Trailing 5-1 in the third set after receiving treatment on a foot injury, Williams seemed set for defeat as her Belgian opponent began to take complete control.

But the American, seeking her fourth straight Grand Slam title, rattled off five games in a row to win 4-6 6-3 7-5.

I just kept fighting and the next thing I knew I had won Serena Williams

Williams, the number one seed, will now face sister Venus in the final as she continues her quest for the 'Serena Slam'.

"It was an unbelievable battle out there," she said.

"I just kept fighting. I thought, 'I don't want to lose 6-1.' Then I said I don't want to lose 6-2. Next thing I know I came back.

"I've always been a fighter. I don't know where it comes from. It's innate."

Williams had struggled at times in the first set and gave it away with two consective double faults.

But she showed the form that has brought her to number one in the world in the second set with an important break in the fifth game to go 3-2 up.

Clijsters had seemed set for victory in the final set

She clinched the set with another break in the ninth game as Clijsters sent a crosscourt forehand well wide.

There was yet more drama in the final set as Williams had to undergo two injury breaks after the third game with Clijsters leading 2-1.

After treatment on a blister on her foot, Williams resumed but her mobility seemed hampered and Clijsters won the next three games to put her in sight of victory.

Williams held serve and then saved two match points - one with a dramatic volley winner - before breaking Clijsters' serve.

Williams held again to make it 5-4 and broke Clijsters once again to level the match.

That's why she's the world number one and why she's won the last three Grand Slams Kim Clijsters

Boosted by the let-off, Williams held serve to go up 6-5 and then broke her opponent to love to win her 27th straight Grand Slam match.

Clijsters denied she had choked in the final set.

"I just kept trying and tried to hang in there... it just wasn't good enough in the end," said Clijsters.

"I wasn't nervous at all, actually. I think she just started hitting winners and much more than in the beginning of the third set when I was 5-1 up.

"That's why she's the world number one and why she's won the last three Grand Slams in a row now."