The sixth-seeded Russian will face number four seed Williams in Friday's semi-final, with the winner advancing to face either number two Jelena Jankovic or fifth-seeded Elena Dementieva in Saturday night's final.

Safina has not played a top-15 player en route to this stage of the hardcourt major, but that will change when she goes head to head with Williams, who has eight Grand Slam titles, including here in 1999 and 2002.

"She fights until the last, until the ball bounces two times on her side," Safina, who is chasing his first Grand Slam title, said.

"She moves very well and she can hit the ball from any position on the court very hard.

"So with her, you really have to be focused until the ball bounces two times or she misses or you hit a winner. That's why with her, you have to be alert every time."

While the 26-year-old Williams is the most decorated player remaining in the competition, Safina has been perhaps the best player on the WTA circuit in recently months.

The younger sister of former men's number one Marat Safin has won three titles and reached the final in six of the seven events she has played since the start of May, winning 37 of her 41 games.

Now Safina is in line to become number one herself if she wins this title and Jankovic - who held the top spot for seven days last month - loses to Dementieva in the semis.

Williams, Jankovic and Dementieva are all guaranteed top spot if they win the tournament.