US Open: Caroline Wozniacki knocks out ninth seed Svetlana Kuznetsova

The scoreline looked standard but the match was anything but: Wozniacki lost the first four games, won the next 10 to claim the first set and stand on the brink of the match, only to double fault to gift a break of serve to the Russian.

But she dug deep and broke again while leading 5-4 in the second set to wrap up victory against an opponent who has caused her serious trouble in recent seasons.

A former world number one, Wozniacki's struggles with form and fitness have seen her plunge the depths - she is currently ranked 74th in the world - and this will be her first progression to the third round of a Grand Slam since Wimbledon in 2015.

"It was not looking good for me," Wozniacki said on the Arthur Ashe Stadium court after the match.

"I've been in this situation before. I was down 6-1 (actually 6-2), 4-1 (in the fourth round in 2009) to her once on this court I said 'I can do this, I can pull this through.'

"And I won last time and I won today and I'm really proud of that."

She will have her eye on a spot in the second week, however: Wozniacki was the runner-up at Flushing Meadows three years ago.

And after posting an inspirational blog via the Players' Tribune website about her parents' sacrifices in the early years, the Dane seems in the mood to finally land the Grand Slam title that her career has always been crying out for.

"I stopped looking at the ranking once I fell out of the top 10," added the 26-year-old Wozniacki.

"I feel like I belong at the top. Today I beat someone who's been playing really well this year and that's definitely going to build my confidence."

Kuznetsova, for her part, admitted that she let her grip slip against an opponent who was there for the taking.

"I made a couple of unforced errors which I shouldn't do and I just let her in the match. And she started to believe," said the Russian.

"I was really frustrated because I felt like I was playing good, just didn't close a few balls at the net."