After crossing the finish line, finding some water, and stopping for photos with a few fans Sunday afternoon, Natasha Hastings was still huffing and puffing and looking for answers.

She’d just won the 300-meter race at the New Balance Indoor Grand Prix in 36.25 seconds, the fastest time by an American woman ever, breaking the previous record of 36.33 seconds held by Allyson Felix — one of the faces of American sprinting.

Hastings was still trying to wrap her mind around the fact that, yes, really, she just did that.

“I really did not come into this race with any expectations,’’ Hastings said in a quiet corner of the Reggie Lewis Center, explaining that her level of recent training wasn’t one that would indicate a record-breaking performance was on the horizon. “I didn’t even have a time in mind. I just wanted to go out and execute a sound race. I was hoping to run nothing slower than 37 seconds. So to look up and see 36.2[5] and then hear American record — I was pretty shocked.’’

Hastings gave some credit to her revamped mechanics. She’s regularly extremely fit, Hastings said, but her technique hasn’t been optimized. She wants to change that.

Sunday was an opportune meet to break it out. The New Balance event is one of four leading up to the IAAF World Indoor Championships in Portland, Ore., March 17-20. It serves as a tuneup and draws high-end track and field athletes from all over the country and, in some cases, the world.

Among them was Meseret Defar of Ethiopia, who won the women’s 3,000 in 8:30.83. It was her first track race in more than two years after she took 2014 off while pregnant and missed 2015 due to injury. She called the win “the biggest race of my life.’’

“I didn’t have confidence to push too much [after so much time away],’’ said Defar, who paced the field by more than 25 seconds. “After the race, I feel my confidence is going up.’’

Abbey D’Agostino, running on an unofficial home track of sorts, finished second in 8:56.77. As a Topsfield native and former Masconomet Regional High standout who also starred as a 12-time All-American at Dartmouth, D’Agostino has raced at the Reggie plenty.

“I have a lot of memories tied to this place, in high school and college — family, friends, extended family here,’’ D’Agostino said. “That’s really cool. The environment does feel a lot different, looks different, just in a different phase of life.’’

On the men’s side, Mike Rodgers won the 60-meter dash in 6.53 seconds, edging 20-year-old Trayvon Bromell’s 6.57, while Vernon Norwood took the 300 in 32.70.

Boris Berian finished the 600-meter run in 1:15.51, the second-best American time ever. Not bad, considering the event wasn’t added to the meet lineup until about a week and a half ago, and Berian had been battling a cold in recent days.

“It was hard, but I’m happy with it,’’ Berian said. “I got better just in time.’’

Other winners included Andrew Wheating (2:18.68) in the 1,000-meter race, New Zealander Nick Willis (3:53.27) in the mile, and Ethiopian Dejen Gebremeskel (7:42.94) in the 3,000. Peter Brady won the masters mile in 4:27.12.

Sam Kendricks won the men’s pole vault at 18 feet, 11 inches, while Omar Craddock took the triple jump with a distance of 55 feet, 2¼ inches. Kurt Roberts’s toss of 70-9¼ won the shot put.

“The crowd was awesome,’’ Roberts said. “They were into it. [This win] sets me up for the bigger picture, which is Rio coming up.’’

Jenn Suhr, who two weeks ago broke her own indoor women’s pole vault record at 16 feet, 6 inches, came up short in an attempt to do it again. She won the event at 15 feet, 9¾ inches, and took a shot at 16-7½, which would have been a world record, indoor or outdoor.

Janay Deloach took the women’s long jump with a leap of 21 feet, 9 inches.

Elsewhere on the track, English Gardner won the 60-meter dash in 7.15 seconds, and Brianna Rollins took the 60-meter hurdles in 7.87. Dawit Seyaum of Ethiopia capped the day with a win in the women’s 1,500, finishing in 4:01.86.

New Balance mixed in a handful of junior races, featuring mostly local athletes, throughout the day. A team from Metropolis Boston was the mixed youth 4?x?200-meter relay in 1:51.21.

Concord-Carlisle (4:10.67) won the girls’ sprint medley relay, with Norton (4:16.79) coming in fourth and Marblehead (4:17.63) fifth. In the boys’ version, Northampton (3:34.75) took top honors, ahead of third-place Needham (3:35) and fourth-place Mansfield (3:35.26).

In the girls’ mile, the Bay State trio of Margie Cullen of Needham (4:50.15), Sarah Edwards of Bellingham (4:51.13), and Sarah Armstrong of Needham (4:51.41) finished third, fourth, and fifth, respectively.

La Salle (R.I.) Academy runner Jack Salisbury won the boys’ mile in 4:08.33, passing Concord-Carlisle’s Thomas Ratcliffe (4:08.74) on the final straightaway.

Tim Healey can be reached at timothy.healey@globe.com. Follow him on Twitter @timbhealey.