Alcaraz is playing on grass for the first time this week since his fourth-round exit at Wimbledon last year and he is slowly getting to grips with the unfamiliar, faster surface.
"I'm happy with the level I played today... I had a really solid match. I'm feeling really comfortable here on grass, getting experience and hours on grass," world number two Alcaraz said.
"Your expectations change when you're playing your first quarter-final (on grass), I'm enjoying every single second and I go into the next round with confidence."
Under sunny skies, Alcaraz raced away into a 3-0 lead in the opening set despite taking a tumble and losing his racket when his Czech opponent fired a winner and eventually got on the board.
But Alcaraz was back on his feet and dusted himself off before quickly pulling away, sealing the first set with an audacious cross-court backhand winner that whizzed past Lehecka as the Czech approached the net and landed right on the line.
Alcaraz carried that momentum into the second set as Lehecka struggled to read his low groundstrokes while the Spaniard also saved three break points to go 4-1 up.
By then the writing was on the wall and although Lehecka valiantly managed to hold serve, Alcaraz sealed the win on his third match point when the Czech's return went long.
Fritz's preparations for Wimbledon took a hit when the American went down in straight sets to Mannarino, the Frenchman winning 6-4 7-6(7) to move into the quarter-finals.
It was a second victory over a top-10 player for Mannarino in as many weeks having beaten Daniil Medvedev in 's-Hertogenbosch.
Mannarino will face Australia's Alex de Minaur who swept Diego Schwartzman aside with an emphatic 6-2 6-2 victory, breaking the Argentine six times and wrapping up the contest in just 65 minutes.Â
