The American appeared in big trouble at the outset of the match as the Belgian smartly changed up the pace of her shots to frustrate the teenager, who was broken for a second time when her forehand went long on set point.

Mertens relished her opportunity to play the spoiler in front of a sold-out Arthur Ashe Stadium crowd firmly in Gauff’s corner, quieting the increasingly tense fans with a shush finger after smacking winners.

After some exchanges of breaks early in the second set, Gauff seized the momentum for good when she broke for a 4-3 lead, pumping her arms and pointing to her ear when her backhand winner levelled the contest at a set apiece.

The third set was all one-way traffic as Gauff’s speedy defence, pinpoint serving, and terrific play at the net overwhelmed former two-time quarterfinalist, Mertens.

Next up for sixth seed Gauff is an enticing first meeting with returning veteran Wozniacki, who came from behind in similar fashion and by a similar score line earlier in the day to see off American Jennifer Brady 4-6, 6-3, 6-1.

“It’s a matchup I never thought I’d have to play,” Gauff said of former world No 1 Wozniacki, who retired in 2020 to start a family and only returned to the game last month.

“When she retired I remember saying that I wish I could have played her. So that wish came true so I’m really excited.”

“I don’t know if it’s a battle of generations,” she said of the matchup between Gauff, 19, and the 33-year-old Dane.

“She’s still fit and going strong. I think it’s going to be an exciting match.”