Great Britain's Laura Robson beat seven-time Grand Slam winner Venus Williams in the first round of the WTA event in Rome.
The 19-year-old Robson, the world number 39, broke the American's serve on six occasions to win 6-3 6-2.
Williams, 32, currently ranked at 24, struggled with a back injury that has limited her to only four other tournaments so far this season.
Match stats
Robson Williams 1 Aces 0 58% 1st serves 53% 8 Double faults 6 73% 1st serve win % 51% 37% 2nd serve win % 34% 6/9 Break points 2/12
Robson will meet Venus's sister Serena, the world number one, in round two.
"I've never played her and I have always wanted to," said Robson.
"It'll be a good test and she is playing probably her best tennis and so she is not losing and so its going to be insanely tough but I'm just going to go out there and do my best."
With the French Open beginning on 26 May, Robson has earned another big win on clay following last week's run to the third round of the Madrid Open.
In Spain, she beat world number four
Agnieska Radwanska, the left-hander's first win over a player ranked in top five, before
losing to Ana Ivanovic in the third round. In that defeat by the Serb, a final-set collapse cost Robson a place in the quarter-finals, but there was no repeat of those nerves in the Italian capital.
The 19-year-old dealt with the blustery conditions, recovering from two double-faults in her first service game to break Williams in the next.
A Williams double fault handed Robson another break and, though the former world number one had 11 break points of her own in the first set, she managed to take only one.
With Williams struggling in the second, Robson opened up a 5-1 lead. And, though Williams briefly rallied, Robson took her third match point with a big backhand.
