The world number one will face second seed Maria Sharapova in Saturday's final after Sharapova had earlier defeated fellow Russian Ekaterina Makarova in straight sets.

"I'm really excited to be in the final again," said the 33-year-old Williams, who is three months older than the previous oldest finalist, 1988 runner-up Chris Evert. "I didn't come here with that expectation so it's really exciting.

"She (Keys) is obviously a great player. I think she is going to be winning this title very soon and lots of other grand slams. I'm really happy to see her do so well and be such a great sport at the same time.

"She pushed me really hard in that first set and I had to really dig deep mentally to get through that."

Keys broke serve in the opening game of the match on her way to a 3-1 lead, only for Williams to break back in the sixth game as the pair traded ferocious ground strokes.

Williams moved 6-3 ahead in the tie-break and although Keys saved the first two set points with aces, she could do little about a service winner from Williams on the third.

Williams, who has won the title every time she has advanced beyond the quarter-finals in Melbourne, took command of the second set with two breaks of serve but Keys bravely saved seven match points on her own serve, while one forehand winner even had Williams applauding.

That meant Williams had to serve out for the match and the 18-time grand slam singles champion did so with her 13th ace of the contest.

Sharapova had earlier made it six wins out of six against 10th seed Makarova, winning 6-3 6-2 in an hour and 27 minutes.

Since saving two match points in her second round match with Alexandra Panova, Sharapova has not dropped a set and has lost just 15 games as she seeks to win a sixth grand slam singles title.

The 27-year-old had to save two break points in a 10-minute opening game and promptly broke Makarova's serve to surge into a 4-1 lead.

Makarova briefly threatened a comeback when she broke back to trail 3-4, only to lose her serve again in the next game before Sharapova served out for the set after 48 minutes.

Two more breaks in the first three games of the second set gave Sharapova a commanding lead and the French Open champion was never in danger of letting it go.

"It's so special," the second seed said in an on-court interview. "When you start off the tournament you take it a match at a time and it's been kind of a strange road to get to the final, but I am here and really, really happy.

"Playing another Russian and someone who was not necessarily the favourite coming into the match, I knew she would come out and play really well and I think I was ready for that, stood my ground and competed well."