Conte led Napoli to the Serie A title last season, while Sarri has just been appointed as new coach of Lazio for the second time.
Hazard, speaking the YouTube channel 'Zack', recalled working with Conte: "When Antonio Conte arrived at Chelsea, it was tough. In the first training sessions, physically, we suffered a lot.
"In the summer we went to Los Angeles. It was hot. We trained morning and afternoon... at the beginning of the season we lost a match against Arsenal. Then everything changed. We went to five at the back. (N'Golo) Kanté became devastating in midfield.
"The training sessions were devastating, but we arrived in the best condition for the matches: that's why we won the championship."
On Sarri, Hazard said: "Sarri arrived the following summer. I knew it would be tough because I knew by then that with Italians... but (Dries) Mertens told me: 'You'll see, you'll have fun, but not in training.' It was true. In training we always repeated the same things, for hours and hours.
"But in the end, that's why we won the Europa League. Personally, I enjoyed myself: I think I had my best season with Sarri."