Attacking her rival Ashleigh Moolman-Pasio (Cervélo-Bigla) with just under three kilometres of the stage to go, the world time trial champion rode to a solo stage victory and increased her lead to 3:35 with only one stage to go.
Speaking to Cyclingnews after the podium ceremony, Van Vleuten said that she felt good on the climb, and had a bit left to attack with.
"I don't think I was ever really in trouble," she said. "Ashleigh Moolman had a good, steady pace, and it was super-hard, for sure. But I felt that I had a little bit left. That's why I went with 3km to go, because I really wanted to win here on the Zoncolan."
With only one stage to go, and with her advantage of more than three-and-a-half minutes over Moolman-Pasio, Van Vleuten thinks that she can rest easy before Sunday's final stage around Cividale del Friuli.
"My team here is so awesome. I didn't have to do anything until the Zoncolan started, so I'm confident going into tomorrow. We did a reconnaissance of the stage, so there will be no surprises for us, and we can take confidence from today's result."
Moolman-Pasio was understandably disappointed at losing out on a historic stage win, but also pleased with her own performance and that of her Cervélo-Bigla team.
"I am proud of my ride today," she said. "Obviously, there is the disappointment of not winning the stage, but I went all-in. I accelerated on the steeper part of the climb, got into a hard tempo, and only Annemiek could follow. I tried to keep that pace, and just before we hit the flatter section, she attacked. It took me a few minutes to consolidate, and I was coming back towards the end, but ran out of climb."
The South African climber acknowledged that she might have won the stage with another strategy, but had no regrets looking back on the stage.
"It is a tricky balance between taking time on the GC from other riders and going for the stage win. I did what I thought was right at the moment. Annemiek was in the lead and did not have to work, so I was driving the pace. My team was incredible, making sure I was in the best position coming in to the climb. All in all, I am proud of the team's effort; I am just disappointed I could not repay them with the stage win."
Spanish champion Eider Merino (Movistar Team) finished fourth on the stage, having been outsprinted by Van Vleuten's Mitchelton-Scott teammate Amanda Spratt in the final metres after an impressive performance on the steep climb.
Merino stayed safe in the peloton until the climb started in Ovaro.
"The race was calm before the climb. Yet at the same time, it was pretty eventful – we had torrential rain, which was so hard that you couldn't even see the road," she explained. "Luckily the roads had already dried out when we got to the Zoncolan."
On the Zoncolan itself, Merino was one of the few riders who tried to chase back to Moolman-Pasio and Van Vleuten after their attack. Even though she never got there and missed the podium by inches, Merino was happy after the finish.
"I felt really good all day, especially on the climb where I tried to pace myself. All in all, I think I spent all energy I had. I kept a small gap over Spratt after I left her behind with three kilometres to go, but Amanda is more experienced than I am, and controlled the situation perfectly. She knew she had to wait until the final 100 metres to leave me behind. I am simply not a sprinter – not even on a climb like this."
The Giro Rosa finishes on Sunday with a 120.3km stage around Cividale del Friuli.