NEW YORK (AP) — Coco Gauff never wavered one bit Thursday night. Not when match point after match point went by the wayside. Not during a 40-shot exchange that ultimately helped decide the outcome. And not, most distracting of all, when her US Open semifinal against Karolina Muchova was interrupted for 50 minutes by environmental activists — one of whom glued his bare feet to the concrete floor in the stands.

It’s been rather obvious for quite some time that Gauff is no ordinary teenager. Now she is one win away from becoming a Grand Slam champion.

Gauff, a 19-year-old from Florida, reached her first final at Flushing Meadows by defeating Muchova 6-4, 7-5 on what was anything but an ordinary evening.

“I really believe that now I have the maturity and ability to do it,” said Gauff, who came close to a major trophy last year in the French Open but lost the title match. “You know, regardless of what happens on Saturday, I’m really proud of how I have been handling the last few weeks.”

The No. 6-seeded Gauff will meet No. 2 Aryna Sabalenka of Belarus for the championship Saturday. Sabalenka beat 2017 US Open runner-up Madison Keys 0-6, 7-6 (1), 7-6 (10-5) in a topsy-turvy second semifinal that finished at nearly 1 am.

Sabalenka won the Australian Open in January, is 23-2 in Grand Slam matches this season and is guaranteed to move up to No. 1 in the world for the first time next week. She was on the verge of losing to Keys, who was up 5-3 in the second set, but used a 12-point run to get back into the match.

Tiebreakers at 6-all in the third sets of women’s Grand Slam matches now are first-to-10 — unlike the old first-to-seven setup — but Sabalenka clearly forgot that. When she moved out front 7-3, she dropped her racket and put her hands to her face, covering a huge smile. Then Sabalenka quickly realized there was work to be done and finished the job on her third match point.

“I was all over the place,” said Sabalenka, who was able to laugh about her mistake afterward.

The toughest part against Muchova for Gauff might have been closing out the victory: She needed six match points to get it done, raucously supported by a loud, partisan crowd that chair umpire Alison Hughes repeatedly implored to quiet down.

After failing to convert one match point while serving for the win at 5-3, then another four in what turned out to be the last game, Gauff got the last chance she would need when she smacked a forehand winner to cap that 40-swing point that was the longest of the contest.

“I knew I had the legs and the lungs to outlast her in the rally; it was whether I had the mentality and patience to do it,” Gauff said. “After 10 or 15 shots in, I was, like, ‘Well, this is going to change the match.’”

She is the first American teenager to make it to the title match in New York since Serena Williams, one of her idols, then quoted another, the late NBA star Kobe Bryant, by telling the fans: “Job’s not done.”