Haas got the better of Pierre-Hugues Herbert 6-3, 4-6, 7-5 and is eagerly anticipating a reunion with his old pal, who is returning to the ATP Tour for the first time since winning the Miami Open in April, after which he made a decision to rest. He stated in an interview recently that he has had enough breaks.
The 39-year-old Haas, who is playing his final season on the tour after a series of injury problems, battled to a 2-6 7-6 (8/6) 6-4 win in just under two hours.
With Nadal ending the clay portion of the season by thrashing Stan Wawrinka to claim his 10th French Open crown, Federer is now preparing to make his comeback as focus shifts towards the grass court swing.
Federer, who fired 23 aces in the contest, had match point at 8-7 in the second set tie-breaker but hit a backhand long.
It means he reaches his first ATP quarter-final since Rome in 2014 and he'll now meet another German, Mischa Zverev.
It was Haas' fourth victory in 17 meetings against the former world number one.
"Playing Roger is very special - especially on grass and also in Germany".
"Deep down I thought there might be a small chance for me if I played well and held my serve".
At the Rosmalen Grass Court Championships, top seed Marin Cilic will meet Janko Tipsarevic in the second round, after the Serbian ousted Yuichi Sugita 7-6 (7-5), 3-6, 6-0. Federer and Haas are good friends off the court, and although Federer seemed stunned after the match, he congratulated Haas.
The contest between the pair was the oldest ATP singles contest since 1982 in Vina del Mar, where Ricardo Cano (30) beat Luis Ayala (49). Roger Federer, bearing a new haircut and the swollen expectations of fans spoiled by his season to date, was shocked by world No. 302 Tommy Haas in the second round of the Stuttgart Open, a tournament with a draw so soft he could have started clearing out shelf space for the title before it even started.