NEW YORK—Early in her first return game against Li Na on Friday, Serena Williams hit a pair of backhands as hard as she could. They went straight into the net. Li took a 40-15 lead, and Serena put her hand out to her side, in her familiar “settle down now” gesture. Sitting a few feet away, I thought it was a little early for her to debut that move. I wondered if we were about to see one of Serena’s occasional panic-button matches, where she fights herself as much as her opponent.

The thought didn’t last long, because Li didn’t give Serena a chance to worry. She double faulted to bring the game back to deuce, and after a few long points she hit a forehand into the net and was broken. But Serena wasn’t out of the mental woods yet. In the next game, she committed her own string of errors and faced a break point—the “settle down now” hand made a second appearance. Like Li, Serena couldn’t find her forehand; she was pushing it and muscling it instead of hitting it. So what did she do? At break point, she a serve that Li couldn’t return. Then she did it again. At game point, Serena did it a third time to hold. It would be a while before we saw the settle-down hand again, as she won eight of the next 10 games.

There are a few things to consider here. The first is the role of Serena’s serve in her success. It probably can be overstated, but it wouldn’t be easy. An anxious Li was broken in her first service game in part because she double faulted at a crucial moment. An almost-as-anxious Serena avoided the same fate a few minutes later because she held the trump card known as her serve. For the match, Serena hit four aces, while Li committed four double faults; that’s a two-game swing right there. Li was able to earn only a single break point all afternoon.

The second thing to consider is the importance of setting in a big match, and this setting in particular. When I hear a commentator say that a player might be intimidated “because he/she has never been in this situation before,” I tend to question whether that’s true. How much do the pros think about it once a match starts? But according to Li, it was very true for her today.

“I was feeling,” she said when asked to explain her poor start, “how do you say, it’s not about the technique, you know. It’s about the problem about myself. I cannot face the problem. First time in the semis. I was thinking about the situation, and I never think about these things.”

“I mean, I shouldn’t be nervous...But when I walk to the court I was feeling the court so big. Even on my side, I was feeling [like it was a] football court.”

Ashe Stadium has been notorious for blowouts this year—of the 41 love sets that have been played in the building since it was opened in 1997, 13 of them have been served up in 2013. It’s hard to think of a reason why this year would be worse than normal, but it is the biggest tennis court in the world, and it obviously takes time for players to feel comfortable at the bottom of that concrete canyon.

The third thing to consider is that series of backhands that Serena hit to start the match. As I said, Williams missed those shots, and at first I thought she was overhitting. But in the end it didn’t matter, and maybe that was the point. Whether she made them or missed them, Serena sent a message with them. Li immediately tried to match her pace, because she must have felt like that was the only way she was going to survive. But it led her to press and make errors—24 of them, against just eight winners. The shanks flew off Li's racquet through the first set and a half, and it wasn’t just because she was playing a semifinal at the Open instead of a quarterfinal. It was because of the woman on the other side of the net.

Serena won a bagel first set, but she didn’t have to play especially well to do it. At the start of the second, some fans were calling out her name and telling her to get going, as if she were behind. It wasn’t until Li finally loosened up late in the second set and raised her game that Serena raised hers. But raise it she did, and the two players gave us a mighty final two games. Their blistering rallies and fierce tension told us: “This is what might have been" Serena was up 6-0, 5-2, but as the game went on and she failed to convert match point after point, she began to play as if it were a third-set tiebreaker.

She fist-pumped. She screamed “Come on!” She screamed a double “Come on!” And, in my favorite moment of the afternoon, she followed a brilliant forehand passing shot by scampering, hunched over and with her fist clenched, halfway across the court. Serena had six match points, but she couldn’t break. In the next game,  she finally suffered the attack of nerves that I thought might come earlier. But she willed herself past it with the power of her shots, as well as the power of her vocal cords. In my notes, next to "30-0," I have “scream” written down. Next to "40-30," I have “crazy scream.”

Serena won that final point, naturally, with a service winner. On Sunday she’ll play her third straight U.S. Open final, which is just what the tournament needed. This has been an event dominated by talk of age, in the case of Roger Federer, and boring blowout matches, in the case of just about everyone else. It’s been nice to have Serena around to defy her 31 years, and to make this blowout something less than boring. She has a way of making a court look very big to her opponents.