NEW YORK — Make way for another young American woman making Grand Slam waves: Taylor Townsend pulled off the biggest victory over her career at the U.S. Open on Thursday, surprising former No. 1 and two-time major champion Simona Halep.

With an entertaining, net-rushing, serve-and-volleying brand of lefty tennis, the 23-year-old Townsend — a qualifier ranked only 116th — came back to edge Halep 2-6, 6-3, 7-6 (4) in the second round at Flushing Meadows.

Townsend was marked for greatness long ago: She was the singles and doubles junior champion at the 2012 Australian Open, turned pro later that year at age 16, then cracked the top 100 in the rankings while still a teenager. But there have been plenty of ups and downs since, including a drop out of the WTA’s top 300, a 9-16 record at Grand Slam tournaments and an 0-10 mark against top-10 foes.

Until Thursday, that is. Halep was seeded No. 4 at the U.S. Open and coming off a title at Wimbledon last month.

“I mean, this means a lot. It’s been a long journey,” Townsend said, her voice cracking as tears flowed. “Just haven’t been able to get over the hump. This match means so much and it gives me so much confidence to know I can do it.”

How did she do it? By moving forward at every opportunity, something so rarely seen these days on tour.