Serena Williams, brimming with confidence and self-belief with every match she plays, stormed into the semi-finals of the US Open today with a 6-1 6-3 demolition of Ana Ivanovic.

The last American left in the tournament after Andy Roddick was sent into retirement by Juan Martin del Potro, Williams was at her irresistible best and steamrolled Serbian Ivanovic in less than an hour.

Williams has won the US Open three times and made the semi-finals in each of her last four appearances at Flushing Meadows but has not lifted the trophy since 2008.

She has been in great form over the past few months, clinching her 14th grand slam title at Wimbledon then winning the singles and doubles gold medals at the London Olympics.

By her own standards, she made a slow start to the US Open but said she was hitting her peak at the right time.

"I feel like I'm going to get more focused and serious and start playing Serena tennis in the next couple of rounds, if I get to play two rounds. That's my goal," she said.

"I didn't think I came into this tournament playing my best, but I definitely played better in the last two matches."

Williams plays Italian Sara Errani in Saturday's (NZ time) semi-finals, a player she has beaten in each of their three previous meetings, with the winner to face Maria Sharapova or Victoria Azarenka in the final.

"She (Errani) is playing well," said Williams. "She made the final at Roland Garros and now semis here, potential finals, so I can't underestimate her.

"It's not by luck she's been doing so well. We will have to see."

Former world number one Ivanovic was appearing in her first grand slam quarter-final since winning the French Open in 2008. She conceded Williams had been too strong but said she was buoyed by her own improved performance.

"Overall I'm very satisfied. I played some really good tennis to get into this position," she said. "Even tonight I felt like I was striking the ball so nicely on both sides.

"That's something that I worked hard for. I think it's a big step for me to reach quarter-final. It's a confidence boost."

Dethroned US Open queen Samantha Stosur has anointed Williams as her undoubted heir apparent, saying the American appears unstoppable at Flushing Meadows.

Williams continued her rampage through the Open draw with a 6-1 6-3 quarter-final destruction of former world No 1 Ana Ivanovic and Stosur can't see anyone denying the 30-year-old a 15th grand slam crown.

The reigning Wimbledon and Olympic champion has conceded just 16 games in five matches.

"Serena, when she's on, is pretty tough to beat. Going on form before this event and so far, it's very hard to go past her," Stosur said before heading to her Florida base for a few days and then home to Australia to recharge for the Asian hardcourt swing.

"I have to say I think she's the favourite to go on and win. She's been playing incredible the last few months."

Presuming she accounts for Errani, who ousted fellow Italian Roberta Vinci 6-2 6-4, Williams will meet either world No 1 Victoria Azarenka or third seed Maria Sharapova in the championship decider.

She leads Azarenka 9-1 head-to-head, has won nine of her 11 career clashes with Sharapova and routed the Russian 6-0 6-1 in last month's Olympic final.

"If anybody's going to beat her, they have to play very well, be aggressive, hit winners, not give her anything," Stosur said.

Although Stosur acknowledged Azarenka as a worthy world No.1, the Australian said Williams deserved title favouritism as arguably the greatest player in women's tennis history.

"If she's not the greatest, she's definitely right up there with the first few," Stosur said.

"To achieve what she's achieved, won all the grand slams in singles and doubles and gold medals and all that, to have the career she's had over such a long time, it is remarkable.

"You've got to take your hat off to it - and to keep coming back year after year after year and be able to perform the way she does, I think she's definitely right up there."

Williams, a three-times champion already, has made at least the semi-finals in her past four US Open campaigns.

Stosur last year, sister Venus in New York in 2001 and at Wimbledon in 2008 and Sharapova at the All England Club in 2004 are the only players to defeat Williams in her 18 grand slam finals.

Still, Williams is still giving Errani - the most improved player on tour and who ambushed Stosur in the French Open semi-finals - due respect.

"She's a great fighter and has a great attitude and believes. I think when you have such self-belief in yourself, then you can do anything," Williams said.

"I love her attitude. I do. I'm inspired by it actually.

"I can't underestimate her. It's not by luck she's been doing so well."

- Reuters, AAP