Reigning Serie A champions Juventus kept in touch with leaders AC Milan with the triumph, which came thanks to two goals in the final 10 minutes from Aaron Ramsey and Cristiano Ronaldo.
AC Milan defeated Torino 2-0 on Saturday, while second-placed Inter Milan was held 2-2 by third-placed AS Roma, with the Old Lady moving up to fourth with their win.
Milan are top on 40 points, with Inter on 37, Roma 34 and Juve 33, although the champions have a game in hand on the rest.
“Today’s victory is an important one,” Pirlo said. “We wanted to keep our run of results going and we won a tough-fought contest.
“It’s what we needed, and we showed desire and character. The fact we’ve closed the gap in the table doesn’t change anything.
“We are Juve and were under pressure to win every game. We’ll see where we end up in a few months.
“The team has grown in confidence in recent weeks, with certain mechanisms becoming more second nature.”
After a scoreless first-half, Juve took the lead in the 50th minute from Danilo but plucky Sassuolo, currently in seventh, leveled in the 58th minute from Gregoire Defrel.
“As for the goal we conceded, we were passive in midfield and our positioning in defence was poor,” Pirlo said. “But Defrel did really well to bring the ball forward with his first touch.”
Juventus are the reigning champions in Italy and have actually won the Scudetto for the past nine seasons.