
The world No 114 regrouped after an error-filled opening set to add the 18th seed to his list of upset victims at Melbourne Park and set up a clash with Novak Djokovic or Alexander Zverev for a place in the final.
Dimitrov looked shell-shocked as the Russian tightened up his game and pounded howitzer shots over the net – faltering on serve and wandering around the Rod Laver Arena court listlessly in the 30°C heat.
The 29-year-old Bulgarian took a medical timeout for treatment on his back after losing the third set but Karatsev would not be denied as he became the lowest-ranked player to reach a Grand Slam semifinal for two decades.