It was the Frenchwoman's most focused display so far, never tipping in to the frustration and ill-temper which almost cost her third round victory over Flavia Pennetta on Saturday. She will play Sabine Lisicki in the last eight.
Bartoli, 26, came into this fortnight on a roll, having won her first career title on grass at Eastbourne and reached her first ever semi-final at Roland Garros. At No.9 her ranking is already equal to its career best, and will improve further next Monday. So whether this result truly amounted an upset is perhaps a moot point.
It is certainly a great result for Bartoli, who had not taken a set off Williams in their previous two encounters. Moreover the only year the Frenchwoman has progressed beyond the last 16 here was 2007, when she was the eventual runner-up to Venus Williams in the final.
The statistics say that Serena had won 24 of her last 26 matches on grass before today. But the fact is Williams returned to action from a foot injury and a life-threatening pulmonary embolism only two weeks ago in Eastbourne, where she promptly lost in the second round. This was only her sixth match in 49 weeks.
Even for a Williams - whose family trademark is playing far fewer tournaments than almost any other players - that is a long lay-off. Serena was clearly not at the peak of fitness here. Her timing and footwork were frequently found wanting.
At 2-1 in the first set loose play from Williams made it 15-40. Bartoli's return deserved the prize but the American somehow got it back. That effort went to waste when a wild forehand saw the ball arc way beyond the tramlines. Bartoli celebrated the break by going through her pre-point rituals with even more energy than usual. All that bouncing around and dancing and stroke practice and fist pumping must have used up valuable energy on a court which was, for once this fortnight, scorching hot.
But for all that mental preparation, Bartoli still required six set point chances before that maiden career set against Williams belonged to her. Three times on Williams' serve she had the opportunity but could not convert. On her own serve Bartoli had to fend off three chances from Williams to put the set back on serve. But at the sixth opportunity, she made hay in the simplest way possible, with an ace.
The second set seemed more concentrated, somehow. Far less ground was given, and for a long time there were no break point opportunities for either player. Serena was focusing on the strength of her serve, setting aside the exchanges when Bartoli's groundstrokes took their toll.
But when she sent a backhand long for break point on 5-5, Bartoli screamed in excitement. The Frenchwoman stepped way inside the baseline to receive Williams' first serve, never mind her second, and in the ensuing rally Serena had the advantage but did not make it count. Bartoli brought up two match points with her eighth ace; on the first Bartoli sent the ball wastefully wide, and on the second the power of Serena's return had the ninth seed stumbling backwards in dismay. Bartoli forced a third, and in a superb battling rally Serena would not be defeated. A great return earned break point for 6-6, then after doing all the hard work and saving it, Bartoli then delivered her fifth double fault of the match for another break point. A killer backhand return took it into the tie-break.Bartoli brought up her fourth match point after a battling rally where Serena somehow seemed to stop halfway through. But it was on Williams' service, and an ace took care of it. An unforced error brought up match point number five, and this time the American could find no reply to Bartoli's serve, sending the four-time champion out of the tournament.