The head-to-head series is tied up at one victory apiece.
The Swiss, who lost his first comeback match in Stuttgart last week, is still lacking some game sharpness and squandered three set points at 5-4 in the first set of an entertaining serve-and-volley encounter.
Federer is into his 11th Halle final and he'll seek a ninth title. In the second round, he beat his fellow German Philip Kohlschreiber. Federer won in Halle on Sunday, defeating #Alexander Zverev in the tournament's final.
The 33-year-old Mayer squandered two break-points at 1-0 in the second set and paid the price as Federer broke again and never relinquished his advantage to win in just over an hour.
On the forthcoming clash with Federer, Zverev said, "Well, it's a final". He beat Yuichi Sugita of Japan in the first round, dropping just four games.
This was a statement match for both of them, brought together for a third time at opposite ends of their careers. He has not dropped a set along his way to the finals and also defeated Zverev's older brother Mischa to make this far into the tournament.
Federer did not face a major challenge in Halle over the last week.
"He's the best of all time on this surface", Zverev said of facing Federer, whom he beat in the semifinals a year ago. Depending on how those ranked above him do at the tournament, Federer might be able to gain a spot in the rankings. But Federer has a lot more experience than his opponent, and is the king of grass court.
It was a gutsy save from Zverev as he sent down a big second serve to save it but a second one quickly followed for Federer, who produced a stunning forehand victor down the line with the German at the net.
Thoughts on the final matchup?