The Basel native, who has won the title seven times, is to face Italy's Andreas Seppi, winner of the second semifinal against Japan's world number five Kei Nishikori.
World number two and top seed Federer needed two tie-breaks to claim a 13th win in 14 meetings with Karlovic.
The Croat, who stands 2.11 metres tall, had battered down 45 aces in his quarterfinal victory against Czech Tomas Berdych on Friday but against Federer he managed a mere 21.
Federer is chasing a 15th title on grass as he warms up for Wimbledon, which begins on June 29th.
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