Trailing 6-4 4-4, Bulgarian world No.10 Dimitrov received treatment on his groin and hip region on Saturday at Pat Rafter Arena.
He went off court and then attempted to play on before retiring to send the 23-year-old into his fourth ATP final.
Dimitrov, a former world No.3, purred to a drought-breaking title a year ago and again looked in fine touch in the Queensland heat.
He now has one week to recover before the Australian Open, while Lehecka is in familiar territory after claiming his first and only title in Adelaide last year.
"I hope it's nothing serious for Grigor; the first set and first half of the second had a good level," he said.
"I'd like to finish the match in a different way, but hopefully he'll be alright and happy to be through.
"Hopefully it can go on and on. I feel great here ... these conditions, they suit me well.
"It's just a pleasure to play here."
Big servers Giovanni Mpetshi Perricard and Reilly Opelka, who beat Novak Djokovic on Friday night, will duel later on Saturday for the right to face Lehecka in the final.
The Czech won't have much time to watch though, scheduled to play a doubles semi-final against Mpetshi Perricard and Nicolas Jarry.
"Seems like they've served a few balls in their lives," Lehecka said of Opelka and the 21-year-old French rising star.
"But doubles this evening will be priority No.1."
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