The sight of a forlorn Coco Gauff hitting serves in the rain on the US Open practice courts became the defining image of her struggles last year.

The serving yips were undermining the two-time major champion’s talent. Staying out in the persistent drizzle was a sign of how desperate she had become to iron out the major flaw in her game.

Twelve months on, the 22-year-old American has returned to the US Open with a rejuvenated service game – and it was the bedrock of her 6-3 6-4 win over Turkey’s Zeynep Sonmez in the New York first round on Tuesday.

Gauff starting with a swift hold to love set the tone for a dominant first set, then used her opening shot to get her out of trouble when Sonmez threatened to go a break up in the second set.

“Now it’s like I can look back a year on and think, ‘Oh, wow, it’s a big difference’,” said Gauff, who is seeded fourth at Flushing Meadows this year.

“It feels good to come into a tournament feeling confident on that shot finally.”

On the eve of her home Slam last year, Gauff decided to reshuffle her coaching team and hired biomechanics expert Gavin MacMillan – who helped four-time major champion Aryna Sabalenka overcome serving troubles earlier in her career.

It was a drastic move. But one which Gauff felt was unavoidable after hitting a high amount of double faults at the preceding Cincinnati Open.

Tweaking technique heading into the US Open – where she receives frenzied attention – was always going to be a risk.

So it was no surprise when Gauff suffered a fourth-round exit in a tournament where she looked fraught throughout and was reduced to tears during matches.

“Obviously last year I knew all the eyes were going to be on the serve,” Gauff said.

“Gavin is so great at what he does, I felt a lot of pressure to kind of prove that. It didn’t really show within the first two weeks, which is normal.”

That was then. Now Gauff possesses a serve which is a serious weapon – even though she still leaks some double faults.

Showing greater consistency on the toss, not throwing the ball too far in front and keeping her wrist straighter have been MacMillan’s pointers for a more reliable serve.

It was a key factor in her charge to the Cincinnati Open title earlier this month.

Analysis from that tournament showed how Gauff is hitting closer to the line but still maintaining her serve speed – underlining a significant improvement in her accuracy.

Gauff now has full confidence her serve can be too hot for opponents to handle – and it showed immediately against Sonmez.

The opening game saw Gauff thump a 112mph ace down the middle, followed by an unreturned first serve and another ace, on her way to holding serve in less than 90 seconds.

Sonmez struggled to get balls into play whenever Gauff landed a first serve in the first set, with the world number four winning 93% of those points.

Gauff broke again for a 1-0 lead in the second set, but her serve was not as effective as Sonmez levelled at 2-2 and held three more break points for a 4-2 lead.

But a precise ace out wide and a 115mph unreturned first serve enabled Gauff to cling on in a game which you felt she would have lost a year ago.

The gutsy hold laid the platform for Gauff to immediately break for 4-3 before securing victory with another service game which displayed her belief.

Gauff hit a trio of unreturned first serves – all just under the 120mph mark, teeing up three match points as Sonmez buckled on the first after a short rally.

“Psychologically, I’m now going to go for the ace, or I know I can hit the serve and the return is probably not going to come back,” said Gauff, who will play Spain’s Paula Badosa or Australia’s Daria Kastakina in the second round.

“Last year I literally was like I hope I make it in. It’s such a big difference knowing almost every serve I’m trying to basically hit 110, 115, 120mph if I can.”