Bayern Munich’s Joshua Kimmich scored a wonderful chip from 20 yards just before half-time was the only goal at Signal Iduna Park.
Bayern are now seven points clear of their second-placed opponents with six matches to go.
“Seven points in six games – it will be very, very difficult,” said Favre.
Bayern are aiming for their eighth successive Bundesliga title but boss Hansi Flick insisted there is still a lot of work to do.
“I don’t live in the future, the here and now is important,” he said.
“We wanted to win and take a big step forward – we did that. We were focused, decisive and courageous at times. I’m satisfied with my team.”
Kimmich’s goal came after both sides had efforts cleared off the line.
Dortmund brought on England forward Jadon Sancho in the second half, but he could not inspire a fightback from his side.
Robert Lewandowski nearly doubled Bayern’s advantage late on but shot against the post. Minutes later, he and the rest of the Bayern players were celebrating what could prove to be a big three points in the title race.
This result continued a remarkable unbeaten run in all competitions that now stretches to 18 matches since they lost 2-1 at Borussia Monchengladbach on 7 December.
Bayern, who are also in the semi-finals of the German Cup and hold a 3-0 advantage over Chelsea after the first leg of their Champions League last-16 tie, have now won 21 of the 24 matches since Flick moved up from assistant to replace the sacked Niko Kovac in November – when Bayern were fourth in the table.
Before the match, dubbed Der Klassiker and played in front of no spectators because of coronavirus restrictions, he said: “Regardless of the result, nothing will be decided.”
However, Kimmich’s goal means Bayern have a commanding grip on the Bundesliga and another title is theirs to lose.