Gael Monfils upset big-serving Canadian Milos Raonic 6-4 6-2 in the first round of the Gerry Weber Open on Tuesday.
Monfils broke serve for 2-1 in the opening set and never looked back.
He also went up a break at the start of the second set and although Raonic put some pressure on him in the final game, he held on before serving out the match with an ace.
"I am more mature after what I went through last year. I've grown up. I try to do simple things."
Once ranked as high as No 7 in the world, the Frenchman missed several months in 2012 because of a right-knee injury that also cut his season short.
Monfils, 26, was ranked No 119 one month ago, but has jumped to No 67 this week after reaching the third round of the French Open.
Monfils said adjusting to grass from clay in Paris was not easy, saying: "It's very hard for my movement. I am more mature after what I went through last year. I've grown up. I try to do simple things."
Meanwhile, Bosnian lucky loser Mirza Basic, playing his first main-draw match on the ATP Tour, stunned seventh-seeded Jerzy Janowicz 7-6 (5), 3-6, 7-6 (5).
The 21-year-old gave himself a match point with a drop volley and won the match when Janowicz reached another drop shot but sent it back long.
Basic was playing only because Philipp Petzschner withdrew with a shoulder injury.
"They only told me at noon that I would be playing. I was a little scared, I was a little nervous, I was playing a big server on grass, on center court," Basic said.
In other first-round matches, Tobias Kamke of Germany beat Lukasz Kubot of Poland 6-4 6-0 and Cedrik-Marcel Stebe of Germany defeated qualifier Jimmy Wang of Taiwan 6-2 6-4.