Guillier "has been a senator for four years and an outstanding journalist, but I have doubts that he has the experience, or the teams, or the program to pull Chile out of the stagnation and frustration we are in today," said Piñera.
"I'm not promising heaven and earth, but I promise that Chile will grow robustly," he said.
Guillier shot back: Piñera "was president of Chile and didn't do the things that we are putting forward. He promised them, but many of them fell apart."
Piñera in turn has to win the 500,000 votes cast for Jose Antonio Kast, his far-right rival in the November election.
Piñera undermined his image of an ex-leader riding back into the presidency by speaking of possible voter fraud in the first round. That generated strong criticism and led Sanchez to say she was voting for Guillier.
To make up some ground, Piñera has vowed to maintain free university tuition brought in by Bachelet for some students, contradicting a previous statement that "free things mean less commitment."