World number six Samantha admits a focused Serena Williams presents a formidable obstacle at the French Open.
Williams is unbeaten on clay this year, having lost just two sets in 17 matches since winning her first clay-court title in four years in North Carolina last month.
It has been a remarkable run of form on the American’s least favoured surface. Williams has won 13 Grand Slam titles but only once at Roland Garros, 10 years ago.
The 30-year-old moved back into the world’s top five with victory at the Madrid Open and the Women’s French Open Betting has her as one of the frontrunners here.
When quizzed on potential winners at Roland Garros Stosur was in no doubt as to Williams’ abilities.
"She didn't do so much through January and at the Aussie Open - I know she sprained her ankle and disappeared again - but then since Charleston she's been almost unstoppable," Stosur said.
"I don't know what it is - she's the only one who can answer that - but she seems super dedicated at the moment, maybe more so than in recent years.
"She's obviously very focused on her own goals that she's trying to achieve and the matches I've played against her, and the ones I've seen her play, she's playing very, very well."
Williams lost to Stosur in last year’s US Open final but warned the Australian she is a different proposition now she is back to full fitness.
"I'm feeling better on clay than I did at the US Open. I have a better ranking and my fitness is better," said Williams.
"I just feel better this time around. I feel I can play on any surface and that's the right attitude for me. I'm enjoying my tennis. This is where I belong and what I do best."
Stosur will be among the favourites when the event gets underway on Sunday with French open tennis odds of around 10/1.