The three-time victor was facing an early exit in her first Slam since the 2017 Australian Open when she trailed Ashleigh Barty by a set and a break, but dragged herself back into form in trademark fashion to progress.
"It's just incredible", said Williams. "I didn't expect to be reading a book about me, that wasn't necessarily true".
"You know, I have cried in the locker room many times after a loss, and that's what I have seen a lot of people do". "Every day is a great day I'm excited to. fight my heart out".
"I think if anything, it shows the passion and the desire and the will that you have to want to go out there and do the best".
But what's typical? Who makes the rules?, " she asked.
Williams once again appeared in her striking, all-black catsuit. For her second match in Paris since 2015, she was erratic throughout, but her iron willpower helped her set up a meeting with Pliskova.
Williams made only three unforced errors in the opening set, 12 in all.
The 36-year-old American, three times a victor in Paris, has lost just two of their 21 meetings and their last-16 encounter on Monday will be among the most eagerly-anticipated of the whole tournament.
Sharapova looked set to notch a third straight win over her rival, but failed to serve for the match twice, with Williams saving three match points on the second occasion.
But she secured a far more straightforward passage into round four with a 6-2 6-1 victory in just 59 minutes.
Williams won the first set with a strong all-around performance.
The big upset of the day saw Czech eighth seed Petra Kvitova eliminated from the tournament by Estonia's Anett Kontaveit.
Sharapova advanced Saturday with a similarly lopsided win hours earlier, beating 2016 U.S. Open runner-up Karolina Pliskova 6-2, 6-1.
While Sharapova elaborated that the two were not friends but they have motivated each other, which is "better than being friends", Williams went on to say that she had no negative feelings towards her.
Champion in 2012 and 2014, Russian Sharapova was refused a wild card for last year's tournament after serving 15-month ban for failing a drugs test, but has looked in ominous form on clay in the French capital.
It was only a brief rally, as Nadal responded in his usual clinical way.
"It has been a really long time", Williams told the Tennis Channel.
The 10th-seeded American beat Camila Giorgi of Italy 4-6, 6-1, 8-6 on Saturday.
Halep has twice lost in the final at Roland Garros, including past year.
The Spaniard, victor in 2016 and seeded third, took just 62 minutes to see off Stosur, the 2011 US Open champ, 6-0 6-2.
Title challengers Marin Cilic and Dominic Thiem struggled into the last 32.