Robson defeats Venus and sets up round with Serena
The British No 1 and world No 41 made light of difficult, blustery conditions to see off the former world No 1, who was plainly struggling with the back injury that has blighted her season so far.
Williams, playing only her second clay-court event of the season following her return from injury, had plenty of chances in the first set and spurned ten break points before taking her 11th when Robson served for the set, only for Robson to immediately respond with her third break to take the opener.
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Hide AdRobson was in dominant form and continued her good form into the second set.
With Williams beginning to struggle, the Londoner looked set to blaze home, breaking twice to go 5-1 up, but had to wait for her victory as Williams briefly rallied to claw one break back.
But there was no repeat of the nerves that had afflicted her when losing to Ana Ivanovic in Madrid last week as Robson took advantage of her third match point chance on the Williams serve with a big backhand to wrap up an excellent win.
Robson will now face world No 1 Serena, who was watching from the stands, in round two.