Pogacar claimed his third stage win of this edition after 177.5 km from Alcossebre to Castello, crossing the line in 4:12:40 with Spain's Mas glued to his wheel after a breathless duel over gravel, summit and descent.
After seizing control of the race with a mountain masterclass in Andorra on stage four, Pogacar was content to sit tight for much of the day while his UAE Emirates XRG teammates calmly kept a 16-man breakaway within reach.
With the category-one Puerto El Bartolo looming, the gap to the leaders was trimmed to 1:27 with 30km remaining. Australian UAE climber Jay Vine then drove the pace in the bunch, steadily chewing into the advantage as the breakaway began to splinter.
Only seven riders remained at the front when Slovenian Pogacar made his expected move with 3.5km to the summit. In a flash, what had once been a two-minute gap evaporated.
As the riders hit the three-kilometre gravel section, Pogacar surged past Belgium's Ramses Debruyne, taking the lead with 23km remaining and 2.5km from the top.
But Mas refused to play the role of bystander. The Movistar rider produced a superb response, overhauled Pogacar to reach the summit first and take maximum points, turning what looked like another one-man show into a proper fight.
The Spaniard then pressed Pogacar on the fast, technical descent towards Castello, where the Slovenian had a scare after drifting off road before quickly latching back onto Mas' wheel.
Their battle continued to the finish, where Pogacar had the sharper sprint to extend his overall lead to three minutes 34 seconds over Mas, who moved above Primoz Roglic into second place. Roglic is third, 4:21 behind compatriot Pogacar.
"They say the heat suited me down to the ground, and that's exactly what happened," Mas told reporters. "I did what I had to do, but Pogacar is just Pogacar ...
"I'm delighted to have finished second here, which means second in the general classification – something I'm very proud of," he added.