PARIS, France - If Sara Errani wants to talk tactics ahead of her French Open semifinal with Serena Williams, she could do worse than having a quick word with Bethanie Mattek-Sands.

Before the start of the tournament, Mattek-Sands shared the practice court with Williams, gaining a brief insight into her preparations and also what it will take to get the better of her.

"We hit for about two hours and she was actually pretty relaxed," Mattek-Sands said. "At the beginning she asked what I wanted to do.

"I said, 'Do you want to play points?' But she was like, 'No, I don't play points in practice. I get too pissed!'

"We both had fun and we were both hitting the ball good and I could tell that she was ready for matches."

So what were the tell-tell signs that Williams, who this fortnight is going for her 16th major, was ready for battle, and what are the keys to ending her 29-match unbeaten run?

"I think it's going to be tough for anyone to beat her," she said. "You have to find someone that can handle the power and play aggressive back to her, because once she kind of takes control of points, she can end them pretty quickly!

"She's playing tough. She hits the ball hard. When I practiced with her, she had a lot on her ball, and she can get a lot on her ball even when she's stretched.

"It's going to take someone to come out guns blazing and playing aggressive. Or if Serena is having an off day, then, you know, maybe that could help too."

As impressive as Williams is technically, it is arguably her mental strength that has kept her alive in the tournament. Trailing 2-0 and break point down in the final set of her quarterfinal with Svetlana Kuznetsova, Williams held her nerve - and serve - to turn the match around.

"She is really strong mentally too," Mattek-Sands added. "You know when it's match day for Serena in the locker room because she's so in the zone. She's looking so cool in there and when it's time to play she's always got her game face on."