Primož Roglič (Jumbo-Visma) was forced to abandon the Spanish Grand Tour after sustaining significant injuries in a crash 100 metres out from the finish line of Stage 16.
Race doctors listed "polycontusions and superficial wounds to his right elbow, hip, knee and ribs" among the Slovenian's troubles, with his subsequent exit confirmed prior to the
162.3-kilometre journey to the Monasterio de Tentudia.
Rigoberto Urán (EF Education-EasyPost) would later prevail in the sprint to the summit, though little pressure was applied to overall leader Remco Evenepoel (QuickStep Alpha Vinyl) who now has Mas as his closest challenger.
Mas made a subtle move during the aforementioned finale but found his effort well-marked by the Belgian, leaving their gap at 2'01" ahead of the final four stages.
Speaking to the media after the race, Mas played down his chances with Roglič out, instead noting his displeasure at the circumstances behind it.
"We’re taking this day by day, and every day is important. Yesterday we all saw Primož crash and every day we have to stay focused,” Mas said.
"For cycling, it’s not nice when somebody crashes. He’s a super rider and a super good person. I think it’s sh*t that Roglič is not here anymore.”
Nevertheless, Mas and Movistar have vowed to push Evenepoel all the way in the fight for the red jersey, perhaps hinting at a potential attack or two to test the waters of his 22-year-old rival.
"To be honest I don’t know what we’re going to do tomorrow," he added.
"But if we have good legs and a good feeling, and the atmosphere in the team is super good, we have to do something, no?
"There really wasn’t much terrain to open up a gap. But I’m pleased with how I felt today and above all with the team. Remco is very comfortable on these kinds of slopes, but I wanted to see how he reacted, and how the rest reacted too.
"We’ve got to keep squeezing him. All the way to Madrid."
La Vuelta continues tonight with Stage 18 LIVE from 10:50pm (AEST) on SBS and SBS On Demand.