Elsewhere, American 17th seed Tommy Paul held his nerve to beat Norwegian ninth seed Casper Ruud 6-2 1-6 6-3 and reach the last four of the California tournament for the first time.
He will face the winner of the match between Russian world number four Daniil Medvedev and Danish seventh seed Holger Rune.
World number three Sinner last tasted defeat in last year’s ATP Tour Finals – where he lost to Novak Djokovic in the final – and has enjoyed a perfect start to the 2024 season.
His winning streak includes a Grand Slam breakthrough title in Australia in January along with a triumph in Rotterdam last month.
In windy conditions, Sinner was in fine form as he broke early in each set and saved the only break point he faced against Lehecka, who was playing in his first ATP 1,000 quarter-final.
“In the morning, it was really, really windy,” Sinner said after the victory. “The first set was tough to handle.
“In a way I am just happy because I made the semi-finals last year and this year I have again a chance to play in the semis in one of the greatest tournaments we have throughout the whole year.”
