The US Open champion was at a loss to explain how he squandered yet another opportunity to win.
"I don't know exactly what the reason was," said Murray. "I have to make sure I tighten that up next week if I get that opportunity in the matches.
"It had nothing to do with my serving today. I missed groundstrokes. Against Raonic, the match points were on his serve so it was a bit different.
"When they're on your serve you want to make sure you're closing them out. It probably takes a bit more focus and, you know, just solid tennis rather than doing anything silly and playing sloppy shots."
Murray believes that Janowicz's 'unpredictability' proved to be the deciding factor in today's contest.
"He's maybe a little bit more unpredictable than a few of them from the back of the court," he added.
"He tried a lot of drop shots and went for winners when he was out of position that maybe some of the others don't."
The 25-year-old enters next week's season-ending finale at the O2 Arena ranked world number three.