Tommy Haas stunned Roger Federer as German players progressed from three of the four second-round matches to be played at the Stuttgart Open on Wednesday.

Federer's long-awaited return to the ATP Tour did not produce the victory the Swiss would have hoped for despite cruising through the opening set.

After 10 weeks off, the 18-time grand slam champion looked understandably rusty, and Haas took full advantage as he battled back to win 2-6 7-6 (10-8) 6-4 - a victory that even surprised the 39-year-old.

"I'm pretty speechless. Playing Roger is always very special, especially on grass. It's tough to put it into words right now, a lot of emotions," he said in his on-court interview. 

"I am obviously very happy about the win, and kind of, sort of, shocked myself.

"He hasn't played a match in 10 weeks. The first match back is always the toughest, so maybe deep down I thought to myself there's maybe a small chance if I can serve well and play well, hold my serve, and that was the case. 

"I didn't really believe it [that I could beat Federer] to be honest."

Haas will face sixth seed Mischa Zverev in the last eight after his straight-sets win over qualifier Yannick Hanfmann.

Germany's third win of the day came as Steve Johnson - seeded fifth - came up short in a thrilling battle with Philipp Kohlschreiber, the American losing 7-6 (7-3) 5-7 7-6 (8-6) after more than two and a half hours on court.

However, Jan-Lennard Struff was unable to make it a clean sweep for the host nation as he lost a deciding-set tie-break against Lucas Pouille.

Struff saved four match points but also let one of his own slip by before Pouille, the fourth seed, found the baseline with a crisp groundstroke to complete a 4-6 7-6 (7-5) 7-6 (10-8) win.