Cirstea, ranked 106th, attributed her loss less to Townsend’s unusual tactics, which she was prepared for, and more to failing to capitalize on opportunities, such as a 3-1 lead she had held in the first set.

“She has a unique game: This is the beauty of tennis,” Cirstea said. “You have to adapt very quick. ”

Townsend was playing in the third round of a Grand Slam tournament for just the second time, after losing at this stage of the 2013 French Open. She told the crowd this would be only the beginning for her, saying she planned to “ride this thing all the way.”

After beating two Romanian players in a row, Townsend will face a third player with Romanian roots in the fourth round: Bianca Andreescu, whose parents moved from Romania to Canada before she was born.

The 15th-seeded Andreescu is also in the fourth round of a major for the first time, after a 6-4, 6-4 win over the 19th-seeded Caroline Wozniacki, a two-time U.S. Open finalist.

Andreescu, who started the season ranked 152nd, has been the most dominant player on tour this year when healthy; her win Saturday improved her record to 39-4, which includes a remarkable 7-0 record against top-10 opponents. She won hardcourt titles at Toronto and Indian Wells, and has not lost a match since March, though she missed several months with a shoulder injury.

Andreescu has little experience on these stages, but said she had learned how to minimize distractions.