PARIS—It’s always been tempting to underrate or even dismiss Bethanie Mattek-Sands as a flake, the Cindy Lauper of the WTA. She’s gregarious, happy-go-lucky, thankful for her lot in life and given to making sometimes outlandish fashion statements—this in a sport where so many of her peers are tight-lipped, aloof, and deeply—some might say tiresomely—conventional. Visor and blond French braid, anyone?

But the reality is that Mattek-Sands has always been a hard worker and a gritty competitor, and today she reminded us of that, beating former French Open champ and world no. 6 Li Na in a second-round match out on the “Bullring” Court 1 at Roland Garros, 5-7, 6-3, 6-2. After it was over, a beaming Mattek-Sands, who wore knee-high, black compression socks because she has circulation concerns, said: “I haven’t worn anything too crazy on court for a few years now. . . I feel like my racquet has been doing a lot of the talking.”

It hasn’t been easy loosening the tongue of that object. Mattek-Sands, described from the early stages of her career as the quintessential, raw, blue-collar player (a rarity in a sport that tends to bleach out what earthiness a player has very early in her career), has battled a series of career-halting injuries, many of which befell her just as she was beginning to make good progress in the rankings.

As late as the summer of 2012, she contemplated quitting the game altogether. Ranked No. 228 and a first-round loser at a small ITF event in Vancouver, she walked off and declared, “I quit. I can’t do it anymore. I’m so frustrated not feeling good and not being able to move the way I want to.” Mattek-Sands’ husband, Justin Sands, listened to her and said: “That’s fine, but you’d better decide if you want to be in or out. Because there’s no in-between. It’s a ‘yes’ or ‘no’ question, and if you want to stay in you’d better be all in. You have to be fully committed, or not in it at all.”

She made up her mind, and while most of the rest of last year was a write-off, she’s off to a great start this season. She’s played a lot of matches, many of them in qualifying rounds, and has shouldered her way back up to No. 61. She says she also feels like a “different person” since she’s taken decisive action to deal with her 26 different food allergies, and she’s undeniably a different player, which you could see today.

Today’s match was a two-hour and 15-minute drama that unfolded in three acts, with two long—really long—intermissions for rain. Li came out in a battleship-gray tennis skirt, and right off the bat her groundstrokes were booming like the big guns on the main deck. She won the first four games, but as sometimes happens with her, she began to misfire. How someone whose game is so well-balanced and grooved can run off the rails so easily is a subject best left for another time.

Mattek-Sands seized the opportunity and fought back to 4-all. After alternating holds, Mattek-Sands’ nerve failed and she hit some errant backhands to give up the critical break; Li then served it out. But Mattek-Sands collected herself for the second set, held serve, and then broke Li. She built a 4-1 lead, and then the rains came.

The women returned after a couple of hours to play just two games—the second of them of them a break by Li that put her back on equal footing at 3-4, with serve to come. But at that point the women were obliged to leave the court and seek shelter once more. Giving up that last break rankled Mattek-Sands, who’s been keeping a journal lately. When asked what she might write in it this evening, she said:

“I’m going to talk about the rain delays and my game plan coming out. The first rain delay she came out on fire. You know, I was up 4-1 and she won two games; got it to 4-3 and it rained again. I was a little pissed at that. I wanted to come out the next delay on fire, just like she had, and I think I did. So that’s one of the things I will write in my journal.”

Sure enough, when play resumed, Mattek-Sands broke Li for 5-3 and was able to make the lead stick, bagging the set with three stunning winners and an unreturnable serve.

Suddenly, Mattek-Sands was on fire. Perhaps that blue-collar background helped. She began to hit forehands as if the racquet were a sledgehammer, and fired backhands as if she were wielding a nail gun. Her serve was up there at a respectable 180 K.P.H. mark (112 M.P.H.) and those point-ending drop shots fell like petals off a daisy.

Mattek-Sands’ concentration was so fierce and her determination so total that her expression wavered between stressed-out and semi-demonic. “I hope I don’t look too stressed out there,” she would say later, laughing. “But I was focused. I know she plays well and I haven’t beaten her yet. So, you know, I was staying focused on my game plan.”

That Mattek-Sands had a game plan at all (topped by a determination to play just as far inside the court as Li) was something of a new development, with as much significance as the marked increase in the sting of her shots, her careful diet, and the decision to keep a journal. All are manifestations of professionalism in a young lady whose goofy gene and general openness have usually created the opposite impression. She’s always been a great fighter, but at 28 she’s also getter wiser.

“I think you can get smarter,” she said. “There is an instance where you might not be trying hard enough, and then there is a point where you are trying too hard. You have to find that happy medium in the middle, and that’s where you play your best tennis. . .

“I just was playing every single point, and I think that’s what I have improved on this year mentally, is—every point is a new point. I have a plan and a purpose each point. You know, I know where I want to hit my serve and where I want to hit my return. I know basically my game plan. I can’t control what my opponent does, but everything that I can control I try and control.”

What Mattek-Sands really controlled in that third set was, true to her plan, the baseline. She took the game to Li and had her back on her heels, and the favorite never had enough time to recover. The atmosphere was intense as Mattek-Sands’ cries of “Come on!” grew more frequent, until she finally finished off Li at the first opportunity, driving a backhand down the line to Li’s backhand corner. Li went for the backhand pass but the ball smacked the tape and fell—on her side.

Suddenly, that stressed, intense expression vanished, and there was the Bethanie we’ve known all these years, her face lit up like that of a happy child on Christmas morning. It was nice to have the old Bethanie back, but the new, improved model is welcome as well.