Serena Williams saved two match-points and came from 5-1 down in the final set to beat Kim Clijsters 4-6, 6-3, 7-5 on Thursday and reach her first Australian Open final in one of the greatest comebacks in tennis history.

"I honestly don't know how I was able to win," a stunned Serena said on court afterwards. "I just thought 'keep fighting'... I really didn't expect to win at that point," she said of the turnaround.

The world number one had looked all punched out against the strong crowd favourite but somehow turned the final set on its head to win six straight games and remain on track to become only the fifth woman in history to hold all four grand slam titles at the same time.

Just Maureen Connolly, Margaret Court, Martina Navratilova and Steffi Graf have achieved the feat and Serena will face elder sister Venus in Saturday's championship decider in a bid to join them.

The final will be the fourth consecutive grand slam the sisters will have contested, Serena having won the 2002 French, Wimbledon and U.S. Open finals.

Venus earlier beat Clijsters's friend and compatriot Justine Henin-Hardenne 6-3, 6-3 on a bleak day for Belgium.

Clijsters had been poised to smash apart the Williams family's grand slam monopoly but wilted at the death.

MOUNTED COMEBACK

Her nerves ragged, she could barely put a ball in court as Serena mounted her comeback.

The final game of the match perfectly illustrated the Belgian's fragile condition as she plopped four consecutive serves out before finally crumbling for good.

It was a phenomenal victory for Serena, though, and one which highlighted her tigerish fighting qualities.

"I just kept fighting and the next think I knew I had won... it was an incredible battle out there," she said.

"I was fortunate -- Kim played so well."

Clijsters, skipping from side to side, got off to a great start, harrying and hustling her way to the opening set with some crashing backhand drives.

The pair exchanged loose breaks at the start of the second set before settling into a rhythm which Serena then smashed with a break for 3-2 in the fifth game.

The American served up a salvo of vicious returns and groundstrokes four games later to break again when Clijsters sent a forehand flying long to hand the set to the American 6-3 after 75 minutes.

A couple of forehand errors allowed Clijsters to break immediately in the third as the momentum took another major shift in this absorbing contest.

She held for a 2-0 lead and when Serena slapped a forehand into the net on the first point of the third game, a plaintive call of "Serena, stop!" escaped from the frustrated player.

WILD FOREHAND

She held serve before receiving treatment for a nasty blister on the ball of her right foot.

The American attracted boos and jeers from the crowd when she immediately took a second three-minute injury break for a 'separate injury' although the trainer continued to work on the same foot.

Clijsters was left to stew for more than six minutes courtside but when she eventually served she held for 3-1 and the crowd exploded.

Serena fell love-40 behind in the next game. She saved the first break point with a drop shot which landed on the net before dropping over but lost the second with a wild forehand.

Clijsters kept up the momentum to take a 5-1 lead as Serena's hopes looked lost.

She staved off defeat by holding for 5-2 and saved two match points in the next game with a big drive from the back of the court and a flying forehand volley, eventually breaking to trail 5-3 but still had much to do.

Holding serve to win her third straight game, Serena heaped the pressure back on the wilting Clijsters.

Two double faults -- only her third and fourth of the match -- put Clijsters on the back foot and nerves strangled the Belgian's chance. She dropped serve again to allow Serena back on level terms.

The top seed held for a 6-5 lead as Clijsters's jangling nerves unravelled and her game fell apart.

Clijsters pushed a backhand long to give Serena three match points in the next game. But the American needed just one to complete a famous victory.

Venus's victory was less dramatic but the elder sibling was no less pleased after finishing Henin-Hardenne off in 74 minutes.

"It's so exciting. You know I've struggled and failed and done everything but get to this position before where I can win the Australian Open," the 22-year-old beamed after a jig of victory on court. "I am just so happy."

Venus's previous best finish here was a semi-final finish in 2001, the year Serena put in her best effort reaching the quarter-finals.

This year one will have her name on the trophy.