Mattek-Sands, whose upset of Li two days ago remains the biggest upset of the tournament so far, looked somewhat out of sorts in the first set, making 10 winners to 14 unforced errors and dropping the set, 6-4. But whatever she told herself at her changeover chair made a massive difference, as she stormed back to win, 46 61 63, making 28 winners to just seven unforced errors in those last two sets.
"I was up 3-0 in the first set and I was playing my game, playing aggressive, making all my shots, and I think she upped her level after that," Mattek-Sands said. "The biggest thing for me is I stuck with my game. Even though I was making some mistakes, I was playing how I wanted to play and I was playing how I knew I could win. Obviously I didn't want to lose that first set, but I was really happy with how I was able to come back in the second set and almost do the same thing. And then after that I got up 3-0 again in the second set and was like, 'All right, this game, right here. This is what I need.'
"I was able to close. It was nice - it was a quick two sets, almost under an hour those two sets."
Ormaechea, Argentina's No.1 player, was playing the third round of a Grand Slam for the first time.
"She had a lot of confidence, coming through qualies and playing three more matches there, and she was feeling good," Mattek-Sands said. "She was ripping the ball heavy today. I know she's had some good results on clay - she made the finals of an event earlier this year and lost to Jankovic in a tough match. I knew she was feeling confident and feeling good and came out with nothing to lose.
"I think she has a bright future. I played her last year, as well, and she has improved a lot."
"I believe I can," the confident American commented. "There are a lot of great players standing in my way. It's going to be a tough road. But, you know, I've got to give myself a shot that I can do it.
"I've dreamed about it. I have seen it in my mind a hundred times. But you don't have everything in your control - you've got opponents that are going to make it tough for you. And there is a little luck involved. But I feel I've worked hard and I've given myself the best chance I can this year."
Also moving through in Victoria Azarenka's quarter was Francesca Schiavone, who followed up her second round upset of No.21 seed Kirsten Flipkens with a 62 61 rout of No.13 seed Marion Bartoli, an upset on paper but not according to the head-to-head - Schiavone is now 6-2 against Bartoli.
"I felt good when I went on the court, but I completely lost my bearings in the match," Bartoli said. "I don't know what happened. The games went by, one after the other, and I found it difficult to get back into it and execute my game, even though towards the end of the match I hung on quite a bit.
"Her game is tough for me - that goes without saying."
Schiavone has now won 18 of her last 20 matches at Roland Garros, and among those 18 wins was a win over Bartoli in the semifinals two years ago - Schiavone would end up falling to Li Na in the final.
Next up for Schiavone is No.3 seed Victoria Azarenka, who beat No.31 seed Alizé Cornet in three sets earlier on (read more here). Azarenka leads Schiavone in their head-to-head, 2-1, though there's an interesting trend - Azarenka won both of their clay meetings, Schiavone their only one on hard.
"Francesca, when she starts winning matches and gets more confident, she's very dangerous," Bartoli said. "Maybe she can trouble Azarenka. And if she makes the quarterfinals, who knows?"