- Khvicha Kvaratskhelia (12')
- Victor Osimhen (33')
Napoli have 62 points from 23 games ahead of second-placed Inter Milan, who host mid-table Udinese on Saturday, and are moving closer to ending a 33-year wait for the league title.
"We won the last few games (before Friday's win), but there was less quality," Napoli manager Luciano Spalletti told DAZN.
"Tonight, on the other hand, in possession, in phase management, in being a team in all things, I felt we were superior to the last games and I can classify that as a great performance."
Kvaratskhelia opened the scoring in the 12th minute after taking possession in midfield and firing the ball into the bottom corner of the net from the edge of the box.
The 22-year-old Georgian, nicknamed Kvaradona, continues to impress this season with 10 goals and nine assists in Serie A. He is now fourth in the scoring charts and has the most assists.
Sassuolo's Armand Lauriente almost equalised two minutes later but his shot from inside the box hit the post.
Sassuolo thought they had cut the deficit just before the end but Lauriente's effort was ruled out by VAR for an offside.
"In my opinion we suffered a couple of times because Sassuolo are well coached and play good football, so you can't have 90 minutes of supremacy and sometimes you have to suffer," added Spalletti.
"We are living a good moment, but nobody has the presumption to go softer and these are important signals for the future."
Giovanni Simeone headed home for Napoli late on but his celebrations were cut short after being ruled offside by VAR.
Sassuolo remain in 15th place with 24 points.
- Andrea Colombo