After thwarting a series of attacks by Quintana on the fifth and final climb of a spectacular stage through the Dolomites, the Sunweb team leader finished ninth at 1min 06sec behind BMC rider Van Garderen to maintain his 31sec lead on the Colombian climbing specialist and a 1:12 cushion on Italy's two-time Nibali (Bahrain).
Bidding to become the first Dutch winner of the three-week grand tour, Dumoulin was buoyed further when Quintana and Nibali failed to counter Frenchman Thibaut Pinot (FDJ), who attacked late to come over the finish just eight seconds behind Van Garderen and close his deficit on third-placed Nibali to just 24secs.
"I was never feeling stressed when they attacked. I knew I had the legs to follow if I needed to," said Dumoulin, who dispossessed Quintana of the pink jersey on stage 10 with a crushing time trial win that left the Colombian nearly three minutes in arrears.
"Vincenzo and Nairo were clearly working together and they lost one minute on guys like Pinot and (Domenico) Pozzovivo.
"I really hope that riding like this, they will lose their podium spot in Milan, that would be really nice, and I would be really happy."
Dumoulin narrowly avoided losing his lead on the 'Queen' mountain stage to Bormio on Tuesday, when an embarrassing, unscheduled toilet stop saw Quintana slash his deficit to just 31secs.
But on another climb-heavy day, the Dutchman reinforced his bid for the 'maglia rosa' with another defiant ride that also pushed Pinot into podium contention.
"If you'd told me last night I'd finish third on this stage, I would never have believed it," said Pinot.
"But I slept really well last night and recovered and so I knew I'd have a good day. Now, I'm going to fight for a podium place in Milan."
On the third of five days in the mountains Dumoulin was only too happy to see an early escape break away from the peloton to go on and build a convincing lead.
It was devoid of any threat to his advantage and allowed him to sit back in the comfort of the peloton.
But with just over 50 km remaining, the tall Dutchman was jolted into action. Quintana raced off the front of their chase group to join Costa Rican teammate Andrey Amador further up the climb.
Alert to the danger, Nibali followed suit moments later, racing ahead of Dumoulin to join compatriot Dario Cataldo.
But Dumoulin, who completed an impressive solo climb over the formidable Stelvio pass following his toilet mishap on Tuesday, was unruffled.
He dug deep and quickly closed the gap.
As Van Garderen and fellow frontrunner Mikel Landa (Sky) forged towards an eventual two-up sprint at the finish, Quintana put Dumoulin to the test again.
But the Colombian's move soon fizzled out, and when Nibali attacked and took Quintana with him with 5km remaining, Dumoulin kept his composure, launching a defiant but brief attack of his own soon after.
Despite his prompting, Quintana refused to overtake Dumoulin and take his turn pulling their group at the front. Not even the threat of losing a podium place to Pinot ruffled Nibali's feathers.
"Today I felt really good," added Dumoulin. "But I'm well aware that I need two more good days."
Friday's 19th stage is a 191km ride from San Candido to Piancavallo.