Jessie Harper felt relaxed when she stepped up to the plate with nobody out and the bases loaded in the bottom of the seventh inning.

On her first pitch, Harper, a freshman, ripped a ball into foul territory. With the count 2-2, recent history indicated that Harper was more likely to strike out than strike a winning hit. Harper was hitless in Arizona’s three NCAA Regional games last weekend, and 0 for 13 in the postseason including 0 for 3 Friday.

Make that 1 for 14.

Harper had the game-winning hit, a walk-off single to center field that scored outfielder Mandie Perez to win the game 3-2, putting Arizona one win from its first Women’s College World Series appearance since 2010.

Oh, Friday was also Harper’s birthday. This beats a cake.

“Tonight was just a great night for the birthday girl here, coming through at the end,” UA coach Mike Candrea said.

Harper had been Arizona’s stabilizing force in the middle of the lineup, behind star slugger Katiyana Mauga, all season long. She’s second on the team with 19 home runs and almost single-handedly helped the Wildcats beat UCLA on the road to win the Pac-12 championship a couple of weeks ago, hitting a grand slam in the first inning. Similarly, Perez, Mauga and shortstop Mo Mercado were all on base, with no outs, that time, too.

In those situations, she’s comfortable, even if she was in her worst slump of the season.

“It started off great with Mandie getting on, and then it just kind of went from there. Kati got on, and then Mo. I was just trying to get the ball up in the air as far as it could possibly go just so Mandie could score somehow just on tagging or a ground ball,” Harper said.

“It was easier knowing they did it (got on base), and all had my back. The fans were amazing. It was just an awesome feeling to do it for the team, and they set it up perfectly where they were able to get on.”

After the game, Candrea went so far as to compare Harper’s demeanor and poise as a true freshman to one of Arizona’s all-time great hitters: Leah O’Brien-Amico, who won four NCAA titles at Arizona. Candrea switched Mercado and Harper in the lineup on Friday, putting Harper in the cleanup spot.

“She’s always got a smile on her face, handles the pressure well,” Candrea said. “It was a challenge for her to struggle a little bit because she really hasn’t struggled a lot throughout the whole year. (She) went back to work during the week and stayed positive, came out tonight and squared one up. … It felt good to see her come through on her birthday.”

The Wildcats were helped by Danielle O’Toole’s performance in the circle. The Pac-12 Pitcher of the Year was stellar through five innings, allowing only four hits and no runs but got into some trouble in the sixth when Kyla Walker led off with a triple and Ari Hawkins followed that up with a two-run home run to left field.

From there, O’Toole (30-4) retired Baylor’s next six batters to finish the game. She allowed six hits overall with four strikeouts and no walks.

“She’s got a great demeanor,” Candrea said. “I’ve never seen her where her emotions were controlling her. She seems to always be in control. Even after she gave up the two runs … she went out and did her job.

“This time of the year, the postseason, you got to be able to do that because crap can happen sometimes.”

Baylor coach Glenn Moore knows. With one out and nobody on in the third, Mauga stepped up to the plate. After the count got to 3-1, the plan was to throw a pitch that Mauga couldn’t hit.

Instead, Baylor’s Kelsee Selman threw it down the middle, and Mauga launched it to right field for a home run, her 25th of the season and No. 92 in her career, putting her just three off the all-time NCAA record of 95. Mauga also scored a run in the first inning on a Mo Mercado double. The Wildcats finished with 12 hits and left seven runners on base.

“Arizona is a great ballclub; give them their respect,” Moore said. “But if we don’t make a mistake pitching, we’d have the two-run home run with a one-run lead at that point. Even though they’ve outplayed us the whole game, we go into the seventh inning with a one-run lead and maybe it changes things a bit. It’s a funny game, and sometimes the team that’s the best, doesn’t win. We’ll come back and fight.”

The Wildcats will try to clinch the best-of-three series on Saturday at 6 p.m. at Hillenbrand.

Contact: zrosenblatt@tucson.com or 573-4145. On Twitter: @ZackBlatt