The match began with five successive breaks of serve – mostly due to excellent returning by both players – but it was Sabalenka who finally took charge.
Doubles specialist Townsend, who walked out wearing a long, black-feathered outfit, continued to frustrate Sabalenka and forced her to serve out the set at the second opportunity, before making the breakthrough in the second set.
But Sabalenka, aware she must lift the trophy to have a chance of retaining her number one ranking, produced an immediate reply as she broke back to love.
That signalled the start of Sabalenka’s dash to the finish line. The top seed lost just five points across the final five games to end Townsend’s challenge in 75 minutes.
Sabalenka was full of praise for Townsend, saying: “What an incredible player she is, approaching the net, a great returner, moving well, very brave.
“You can see how focused she was. It was really challenging playing her, especially here.
“Thank you so much [to the crowd] for being so respectful to me. I knew coming into this match that you guys would be on her side, but you were sometimes cheering for me and I really appreciate that.”
At home on the US Open hard courts, Sabalenka has not failed to reach the semi-finals here since 2020.
But, following a fourth-round exit at Wimbledon, she is aiming to avoid a first season without a major title since 2022.
Noskova’s sense of relief was cleared as she crouched to the ground after taking her sixth match point about an hour after earning her first.
The 21-year-old served for the match in the second set, but missed her first two match points as Kostyuk battled back from 5-3 down to force a decider.
The players exchanged breaks of serve in a tense final set, but it was Noskova who emerged, once again, with a 5-3 advantage.
Noskova failed to take her next two match points as a couple of misses helped Kostyuk to deuce, and missed her fifth after saving a break point, before finally getting over the line after an emotionally draining two and a half hours on court.
“I am more than relieved, this match was so tough, emotionally a bit more than physically,” Noskova said.
“Today was very close, it could have ended both ways. With all the chances I had in the second set, it was tough to still kind of restart.”