The World Championships time-trial bronze medallist from last year shredded the field on yesterday’s 17th stage of the race, a 38.7-kilometre individual time-trial which started and finished in the town of Burgos.
Having started the stage fourth overall, 1’51” seconds down on home favourite Joaquim Rodriguez (Katusha), Dumoulin clocked a time of 46:01 for the test against the clock, over three minutes faster than the Spaniard who dropped to third overall.
Describing the time trial, Dumoulin said: “I was mentally in a good place; in the morning I did a second recon of the the course and I asked my sports director to note down every little detail I needed to know to ensure I produced the best performance.
“I was very focused and had really good legs. That’s what made the difference.”
Looking into the future to two tough mountain stages with and such a narrow advantage on second placed Fabio Aru (Astana), Dumoulin was more than realistic about his chances.
“I’m really happy, I did a very good time trial, but the race is definitely not over.
“Three seconds time difference is virtually nothing.
“It’s going to be a real spectacle for the spectators but one hell of a close fight.
“There will be difficult days ahead and Aru’s definitely going to try to get me out of this jersey.”
Ireland’s Nicolas Roche was 49th at 3:44 and is now 30th at 46:15.
A fascinating conclusion to the race is in prospect as Dumoulin seeks a momentous moment for Dutch cycling.
It is 35 years since a Dutchman won a Grand Tour, when Joop Zoetemelk won the 1980 Tour de France. He also won the 1979 Vuelta.
Today’s 204-kilometre stage from Roa to Riaza has two categorised climbs in the first half of the race followed by a nasty category one ascent with 15 kilometres to go.
Meanwhile, Ryan Mullen was once again best of the Irish on stage four of the Tour of Britain, the Garmin Cannondale man crossing the line in 61st place in the same time as stage winner Fernando Gaviria (Etixx-QuickStep).
The Colombian beat Andre Greipel (Lotto Soudal) with Edvald Boassen Hagen (MTN-Quebeka) in third.
The other Irish riders were Eddie Dunbar (NFTO) and the An Post Chain Reaction pair of Conor Dunne and Jack Wilson.
Dunbar was 76th at 46 seconds while the latter two were 85th (at 1:27) and 93rd (at 2:49) respectively.
Spanish rider Juan Lobato (Movistar) leads Boassen Hagen by six seconds overall.