The 175.8 stage from San Lorenzo de El Escorial and Cercedilla was the final general classification showdown before the finale in Madrid.
The stage was won by Lampre-Merida's Ruben Plaza after he mounted a mammoth solo breakaway 118km from the end of the stage. However, Plaza's win was overshadowed by the GC battle going on behind him.
The burning GC question was whether Dumoulin (Giant-Alpecin) had the legs to hold on to his six-second lead over Aru (Astana).
That answer came a little under 50km from the end of the stage, after Aru mounted a raid on Dumoulin's position over the penultimate climb. Aru's attack, which was swiftly followed by the other GC contenders, cracked the race leader. The move isolated Dumoulin and saw him dropped entirely off the podium by the finish in Cercedilla - seeing him fall to sixth on the general classification, 3min 46sec behind Aru.
“This victory is thanks to my teammates, it’s not just about me,” Aru said. “They’ve been phenomenal for the whole Vuelta – Mikel, Luis Leon and Dario Cataldo – and we think of the guys who aren’t here, Paolo Tiralongo and Vincenzo Nibali.
"We had our difficulties at the start of this race but we’ve worked hard and we’ve been very united."
Dumoulin was disconsolate at the end of the stage.
“I was just empty, I had no legs. I had an idea that was the case but I just fought for what I was worth and in the end you just got to deal with it,” Dumoulin said. “Tomorrow I will feel proud but today it’s just disappointment.”
From the point Dumoulin was dropped, the general classification became a race against time with a major shakeup on the cards.
Aru now leads Joaquim Rodriguez (Katusha) by 1min 17sec with Rafal Majka (Tinkoff-Saxo) a further 12 seconds behind in third. Nairo Quintana (Movistar) made a valiant attempt to reach the podium but fell just short to end up fourth.
Esteban Chaves (Orica-GreenEDGE) jumped two places on the stage from seventh to fifth after another composed ride.
Plaza won the stage after a monumental effort which saw him launch a 118km long solo breakaway from the second climb of the day and hold it to the finish.
The Spaniard, who also won a stage solo at the 2015 Tour de France, was never headed despite a desperate chase by his former breakaway companions José Gonçalves (Caja Rural-Seguros RGA) and Alessandro De Marchi (BMC).The three riders finished the stage in that order.
“I tried to get away in a smaller group over [the first climb] of Navacerrada and when that didn’t work out, I went for it as soon as we started [the second ascent] of La Morcuera." he told biciciclismo.com.
“When I went for it, my aim was to make it all the way to the finish. I know these climbs well,” Plaza added. “It was all about being consistent and I had to calculate my strength right down to the last gram of energy. This is the last stage of the race I could possibly win, and I said it was all or nothing.”