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The hosts opened the scoring soon after though as Ante Rebic unleashed a first-time shot from the edge of the box, which went in off the inside of the left post.
Fiorentina continued to press as half time approached, adding a fourth shortly before the interval as Mario Suarez rifled the ball in from the edge of the box.
The hosts were on the charge again early in the second half when Raman Chibsah gave away a free kick moments after the restart, but Fernandez's ball forward failed to result in a shot.
Valero kept the pressure on, lofting the ball into the penalty area, but the visitors were able to clear, while Fernandez saw a through-ball go straight out for a goal kick.
Giuseppe Rossi came off the bench in the final half hour, giving the hosts some fresh legs going forward, but Samuele Longo blocked his first attempt, before a second a couple of minutes later went just wide.
Having rarely found themselves in the Fiorentina box throughout the first 75 minutes, the visitors tried to finish the match on the front foot, but Daniele Verde unleashing a sliced effort from out wide, only to send the ball over.
Frosinone's late resurgence resulted in a consolation goal with a couple of minutes to spare as Alessandro Frara got the final touch in a crowded penalty area following a lengthy buildup.
The result moves Fiorentina back to the top of the Serie A table, while Frosinone sit 17th, four points clear of the relegation zone.