Serena Williams knew she needed something extra against Simona Halep, and she found it

NEW YORK—“I knew if I wanted to win this,” Serena Williams said after her 6-2, 4-6, 6-3 quarterfinal victory over Simona Halep on Wednesday, “I really had to step it up in the third set.”

Unlike most of us, when Serena thinks something on a tennis court, she can usually be counted on to make it happen. By the end of the second set, Halep had built up a head of steam; serving at 5-4, she survived an old-fashioned onslaught from Serena and saved four break points to close out the set. But Serena wasted no time stopping Halep’s momentum cold at the start of the third.

At 1-2, with Halep serving at break point, Serena hit a strong forehand return that won her the point and the game. She had been on the verge of breaking Halep in the second set, but hadn’t quite gotten over the hump. Now she had, and a match that had been spectacularly competitive 10 minutes earlier, that had fans gasping at the speed and quality of the rallies, suddenly felt like it was all over.

When Serena said she had to “step it up” in the third set, though, did she actually mean that she needed to take it down a notch? She had broken out of the gates hard in the first set; by the six-minute mark, she had hit five aces and seven winners and was up 3-0. But soon a shank or two began to seep through the cracks, and by the second set that trickle had turned into a flood. Halep finally got her foot in the door when she held serve at 0-1 in the second set, after saving seven break points. But Serena did her part to open that door in the first place; she lost five of those break points on backhand errors.

Once Halep had survived Serena’s final, sustained act of aggression at 5-4 in the second, Serena knew that the mistakes had to go. And so they did, as Serena basically tightened everything up in the third. She would make 43 errors in the match, but just nine in the final set. Her first-serve percentage rose, but her ace percentage fell. Her net game became more efficient; she won nine of 11 points there in the third set. And while she cut down on the errors, she also hit fewer winners; Serena had 50 for the match, but just 10 in the last set. She was also effective with the ultimate control play, the drop shot, and she kept her fist pumps and “Come on!” screams to a minimum.

In many ways, this match reminded me of the more competitive contests between Serena and Victoria Azarenka. In those, Serena would dominate for a stretch, only to grow erratic and watch as Vika fought back. Then Serena would stop missing; it was almost as if she could have played a more slightly conservative game all along and won easily, but what fun would that have been? More remarkable, in those matches and on Wednesday night, was the way that Serena could seemingly will herself to do this in the middle of a match.

Or maybe that’s just how she woke up on Wednesday.

“I was really rather calm today,” Serena said, sounding puzzled by her own mood. “...I guess that’s just how I got out of bed this morning. I definitely wanted to do some more ‘Come ons.’ I don’t know why. That’s weird.”

Serena admitted that, while the way she won may have been the right way, she would still rather avoid it if she could.

“I don’t really like coming to the net, to be honest,” she said, “but I’m good at the net, I guess. I’ve got to do what I’m good at.”

Credit Halep for pushing Serena to do what she didn’t want to do, in a way that only Azarenka has in recent years. Halep started tentatively, but by the second set the two women were trading blistering ground strokes and skating across the baseline to track them down. In these points, Halep gave as good as she got.

“I think this was the best match that I ever had against her,” said Halep, seeming to forget that she had once beaten Serena, 6-0, 6-2. “I felt that I’m very close and I have chances to win ... I take it, like, with confidence, this match.”

Still, Halep, as I wrote earlier in the tournament, was playing uphill the whole time. Or, to be more precise, she was starting a shot behind. Whatever happens from the baseline between these two, the serve will always be the difference-maker. Serena had 18 aces; Halep had one. Serena won 51 percent of her second-serve points; Halep won 38. Thirty-seven percent of Serena’s serves didn’t come back, compared to just 17 percent of Halep’s. That’s the tennis version of having a great first step. Two decades after her first appearance at the Open, it still has Serena in front of the pack.