He won 3-6 6-4 6-2 in Stuttgart after sitting out the clay-court season.

"Three months [out] is a long time, it's longer than the year-end break," said Federer.

"I'm very happy returning on good terms on to the tour. It's difficult, missed some chances maybe in the first set, he connected well at the right times. Then I found my rhythm."

He added: "I'm super happy to be back on tour - it's been a great start."

During his two and a half months out, Federer has dropped to second in the world rankings, behind Spain's Rafael Nadal.

But the 36-year-old will regain top spot if he reaches the final in Germany and faces either Guido Pella or Prajnesh Gunneswaran in the last eight.

• Live scores, schedule and results
• Alerts: Get tennis news sent to your phone

Federer, who has won 20 Grand Slam titles, was playing for the first time since losing to Australia's Thanasi Kokkinakis in the second round of the Miami Open on 24 March.

The Swiss, who had a bye in the first round, made a poor start against German Zverev, 30, losing his first set on grass since a defeat by Tommy Haas in this competition last year.

But the reigning Wimbledon champion took the second set and broke Zverev's serve twice in the deciding set to move into the quarter-finals.

This is only the fifth tournament Federer has played this season. He won the Australian Open in January, the ABN Amro World Tennis Tournament in Rotterdam in February and reached the final of Indian Wells in March.

Federer has won Wimbledon eight times and this year's tournament starts on 2 July.

India's Gunneswaran, ranked 169th in the world, caused an upset on Wednesday with a 7-6 (8-6) 2-6 6-3 victory over sixth seed Denis Shapovalov of Canada, the world number 23.