The Belgian was in scintillating form as he moved up to lead the Premier League's goalscoring charts with five goals in as many games - despite beginning two of those on the bench.
Hazard was very strongly linked with a move to Real Madrid in the summer, but with any interest turned away by Chelsea, he has started the season in fine form - and he is already almost half-way to matching his goal tally from last season.
Chelsea were 4-1 winners over Neil Warnock's Bluebirds but it took time for them to get into their stride and keep up their perfect start to the season, which now extends to five games.
Chelsea trailed for nearly 20 minutes after Sol Bamba turned in Sean Morrison's flick from a free-kick, but never looked back once Hazard scored two quick-fire goals before the break to put them in front.
"We did not play very well in the first 15 minutes," Sarri said. "We knew they were very dangerous from set-pieces, with long balls, but we played much better towards the end of the first half, and in the final half hour. Every day I think we need to improve - and we can.
"Of course, our results have been perfect, but we can do more with our performances. I think we are very dangerous going forward, but not so solid defensively."
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