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Paul delivered a resilient performance on the clay to defeat the world No.9 2-6 6-3 6-3 in two-hours and 16 minutes.

It was a huge disappointment for De Minaur, who appeared to have put his recent woes on clay behind him with stirring wins over Luciano Darderi and Cameron Norrie.

But defeat to Paul adds to a list of losses to lower-ranked opponents, having also lost to Rafael Jodar and Valentin Vacherot leading into Hamburg.

It was only Paul's second win over De inaur in seven meetings, and after the first set, it looked as if he was on his way to another defeat.

"I was in panic mode, down a set and 3-0," Paul said.

"I thought he was playing amazing tennis and I was just trying to find ways to win points. I feel like I stayed resilient, put balls in, made points longer and I started finding my forehand. I shifted it and started playing some really good tennis."

In the other semi-final, world No.57 Ignacio Buse needed just 64 minutes to dispatch American Aleksandar Kovacevic 6-1 6-4.