The Quick Step-Floors rider clocked a best time of 21 minutes 39 seconds to beat twice Tour de France champion Alberto Contador (Trek Segafredo) of Spain by 19 seconds, and another Frenchman, Tony Gallopin (Lottos Soudal) by 20 seconds.
Alaphilippe, second in the top classic Liege-Bastogne-Liege in 2015, winner of last year's Tour of California and fourth in the Olympics road race, now leads Gallopin by 33 seconds and Spain's Gorka Izaguirre (Movistar) by 47 seconds.
Among the top climbers still in contention, Colombian Sergio Henao (Team Sky) is fourth overall, 1:05 off the pace ahead of Irishman Dan Martin (Quick Step Floors), who is 1:20 behind the leader.
"I was very, very motivate" said Alaphilippe, looking to become the first Frenchman to win Paris-Nice, one of the top week-long stage races, since Laurent Jalabert prevailed in 1997.
"I was very nervous but I felt my legs very good during the warm-up. Nothing is won yet, a 1:30-lead (over Contador) is nothing, but I will not surrender."
Contador, who is in eighth place, 1:31 off the pace, said: "I rode a good time trial, but Alaphilippe was really fast, especially on the flat part before the climb up the Mont Brouilly. Congratulations to him."
The Spaniard will bid to overturn the deficit in Saturday's penultimate stage which finishes up the Col de la Couillole, a 15.7-km climb at an average gradient of 7.1 per cent.
"It was a very fast time trial, it's now time to recuperate to be ready for the next stages," said Contador.
"Alaphilippe and Gallopin are the two favourites. I’m behind, I’ll see what I can do but the truth is that it will be very complicated. Alaphilippe is extremely talented."
Thursday's fifth stage takes the peloton from Quincie-en-Beaujolais to Bourg-de-Peage.
(Reporting by Julien Pretot; Editing by Ken Ferris)
Share
