World number 88 Petkovic was the better player in the biggest points, breaking the twice Grand Slam champion three times and committing 15 fewer unforced errors on a sun-soaked Louis Armstrong Stadium court.

The 31-year-old German, who celebrated the win by lifting her shirt and pointing to her abs, said she is in a great state of mind despite winning only eight main-draw matches this year.

“In the past two weeks I’ve been really good with my mental state,” Petkovic said.

“I’ve been trying to stay in the moment and play point by point, enjoy the time I get to spend on this beautiful court and it’s been really working for me, so I’m glad.

“There were ups and downs throughout the match but I was there in the most important moments.”

She tipped her hat to the Czech, who has been hampered this summer by a left forearm injury.

“Petra is one of my favourite players and people on Tour,” she said. “It’s always so tough when I play against her and I’m so happy.”

Petkovic will be the underdog again when she takes on 25th seeded Mertens, who trounced Kristyna Pliskova 6-2 6-2.