Tied at one set apiece, the Pole looked in control, annoying the Briton with a combination of powerful ground strokes, slices and drop shots.
But it all went wrong.
The 22-year-old Janowicz, lost five consecutive games and the match, 6-7 (2), 6-4, 6-4, 6-3.
“Such a shame I didn’t play my best tennis today,” said Janowicz. Even the 30-minute interruption of play to shut the Centre Court roof could not help him.
Murray said he was surprised by Janowicz during the break, spotting him in a conversation on his phone.
Still, Janowicz looks like he has a brilliant future ahead of him.
“He’s obviously young. He has a huge game,” Murray said.
The handling of nerves was a big theme in both semifinal Men’s matches.
In the fourth set, with Juan Martin del Potro looking gassed, Novak Djokovic broke to lead 4-3.
Things were looking bleak for del Potro, but he summoned something extra to break back.
“It was about the nerves, and Nole handled it much better,” said Djokovic’s coach, Marian Vajda.
Djokovic got the last break he would need when del Potro sailed a running forehand long to make it 5-3. “I know ... I have been pushed to the limit today,” Djokovic said. “This is where ... also mental ability to stay tough, can decide the winner.”