The Frenchman, who claimed his maiden stage victory at this year's edition of the race held for the first time in Saudi Arabia was four minutes and four seconds faster than Alonso on Monday, reports Efe news.
"I am absolutely delighted. It is a wonderful story," said Serradori, dedicating his victory to Portuguese motorcycle racer Paulo Goncalves, who died Sunday in an accident during stage seven of bikes.
Alonso said he is "happy to complete the day without any issues. It was a beautiful stage, quite fast with a lot of dunes, so it was good.
"I think the dunes are harder than tracks, but for whatever reason I feel more comfortable...I am enjoying so far this second week of the Dakar," the two-time Formula One champion added.
Argentine X-raid driver Orlando Terranova came third, followed by South Africa's Giniel De Villiers and Saudi Yazeed al-Rajhi, respectively.
Meanwhile, Spanish Mini driver Carlos Sainz crossed the finish line 15th to be behind Alonso for the first time in this year's race.
Although he was 19 minutes and 15 seconds off the pace, Sainz held onto the top spot in the general rankings of the Dakar car race.
The eighth stage of the rally in the bikes and quads categories initially scheduled for Monday was cancelled following the death of Goncalves.
This marked the first death of a Dakar Rally participant in five years. The last fatality was Polish motorcyclist Michal Hernik, who died of dehydration.
( With inputs from IANS )