Stosur and Venus battle in Madrid
US Open champion Stosur was forced to save six break points as she battled through the clash 7-5 4-6 7-6(5) to secure her place in the second round.
“Petra was playing really well, and it really went down to the wire today,” Stosur said. “I thought my level got better and better as the match went on, and the last half of the third set was the best tennis we played.”
Next up for Stosur will be Christina McHale, the American beating Sweden’s Sofia Arvidsson 6-4, 7-5.
“She’s an up-and-coming young player who hits the ball well and is a great athlete,” Stosur said. “I will have to be prepared and be on my game.”
Williams, losing finalist in Madrid two years ago, also dropped the second set before rallying to beat Romania’s Simona Halep 6-1 4-6 7-6(6).
“On the last match point I went for it. I wanted to leave it all out there on the court,” said Williams.
“A match like that can be good. We hit a lot of balls and I was able to get that clay-court mentality right away. I have to keep remembering I’m on clay.”
French Open champion Li Na eased past Madrid teenager Garbine Muguruza 6-2 6-4 while unseeded German Mona Barthel, who has been enjoying a great season including winning the title in Hobart, beat compatriot Julia Goerges 6-4 6-1.
"This time last year I was ranked around 200 and now I'm close to being seeded," said Barthel, making her Madrid debut. "It feels strange, but I worked hard for it.
"I'm serving well and moving better on the clay and I'm obviously benefitting from confidence right now."
Seventh seed Marion Bartoli did not make it through to the second round, however, when she was beaten 6-7(8) 6-4 6-3 by former French Open quarter-finalist Sorana Cirstea.