The top-ranked Federer looked rested and refreshed after last playing Sept. 20, when he helped Switzerland beat Italy in the Davis Cup.
"I was eager to come out and play again," Federer said. "It's especially nice to come back on tour with my home tournament where I have so many memories."
Federer broke Rochus' serve in the sixth game of the first set and fifth game of the second.
"I thought it was a good match for me. At key moments when I needed a serve, it was there," Federer said.
Federer is chasing a fourth straight title at the euro1.75 million ($2.58 million) Swiss Indoors, where he once worked as a ballboy.
He will next play 51st-ranked Andreas Seppi of Italy, who rallied to beat Swiss wild-card entry Stephane Bohli 4-6, 6-1, 7-5.
Federer showed a little rustiness early on but created a break point with a forehand crosscourt winner past Rochus, with whom he won the Wimbledon boys' doubles title in 1998.
Federer's blistering forehand winner at 2-2 in the second brought another break point, which he took when Rochus netted.
Rochus saved a match point when Federer netted a forehand volley, but the defending champion took his second with a powerful serve that the Belgian returned wide.
Federer is seeded to meet No. 3 Novak Djokovic in the final. The Serb will play Germany's Andreas Beck on Tuesday.
France's Florent Serra, a lucky loser from qualifying, survived a barrage of 28 aces to beat Croatia's Ivo Karlovic 7-6 (5), 6-4.
The 2.08-meter (6-foot-10) Karlovic has an ATP tour-leading 855 aces this year, but his second serve failed and he couldn't break Serra.
The 71st-ranked Serra will next meet fifth-seeded Radek Stepanek of the Czech Republic, who beat Germany's Rainer Schuettler 6-2, 6-0 in the late match.