Demichelis, who refused to sit on the bench early in the season after being dropped, came on just before the start after central defender Holger Badstuber was injured during the warm-up in Munich.
The Argentine made the best of his rare start to glance in a header from a Danijel Pranjic cross six minutes before the break to give injury-plagued Bayern the lead after a lacklustre performance for much of the first half.
Coach Louis van Gaal heaped praise on the 29-year-old, who has said he was willing to move if he continued to be overlooked in his eighth season at Bayern.
"Martin showed again today that he is an outstanding central defender. He has never said he wants to leave and I have also never said it. We can talk about it when the time comes but now is not the time to do it.
"In the second half my players showed how Bayern should be playing," the Dutchman told reporters.
Bundesliga top scorer Papiss Demba Cisse could have given the visitors an early lead in the 16th minute but his header at the far post sailed just wide.
Bayern, missing half a dozen key players through injury, responded with Mario Gomez chasing down a Toni Kroos through ball but his shot was off target.
Gomez made amends in the 61st minute, heading in another Pranjic cross for his fourth goal of the season, but Freiburg substitute Stefan Reisinger briefly cut the deficit three minutes later.
Ukrainian Tymoshchuk, who has hinted he would like to leave in the winter transfer window if he did not get to play regularly, fired in Bayern's third from the edge of the box and Kroos added another with a thundering long-range effort nine minutes from time.
Edson Braafheid's own goal in the 86th minute did little to dampen the champions' celebrations for their fourth league win as the Bavarians moved up with 15 points.
