Hennemann qualified for his only Grand Slam tournament at the 1988 French Open, where he beat fellow qualifier Stéphane Grenier in the first round. He was beaten in the second round by Thomas Muster.

He was more successful on the doubles circuit than as a singles player, winning two Challenger doubles titles.

The Novo Hamburgo born player was a doubles semi-finalist with Ivan Kley at the 1987 Athens Open and a doubles quarter-finalist partnering Jose Clavet at Palermo in 1988.

Challenger titles

Doubles: (2)

No.	Year	Tournament	Surface	Partner	Opponents in the final	Score in the final
1.	1988	Salou, Spain	Clay	Jean-Marc Piacentile	Scott Patridge Otis Smith	6–4, 6–1
2.	1989	Sao Paulo, Brazil	Clay	Edvaldo Oliveira	Nelson Aerts Alexandre Hocevar	6–3, 6–3