Valverde lost time to Bernal when the Colombian won an intermediate sprint, with his overall advantage dropping to 16 seconds.
However, on the final unclassified climb of the stage, after numerous attacks by GC rivals including Bernal, it was Valverde who soared over the summit at the head of the main pack.
Finishing eighth on the stage, Valverde now has put the hardest days, in terms of terrain, behind him. But with Bernal keen to keep chipping away at his lead and less than 20 seconds between the two top racers, the Movistar man said afterwards that it remained far from certain he would win his third Volta a Catalunya on Sunday in Barcelona.
"The toughest stages are done with, but there are some key stages left to come," Valverde said. "[Saturday] they're forecasting rain, and that can make the stage even harder."
Stage 6 from Vielha to Torrefarera is 194.2 kilometres and takes the Volta a Catalunya southwards out of the Pyrenees. It features three second-category climbs, the first as soon as the stage begins and the third some 70 kilometres from the finish.
The Volta is a race that has been won by considerably less than Valverde's current margin in the past, and, as he said, "Sixteen seconds is a lot and not very much at the same time." In 2009, Valverde finally won the Volta by 15 seconds over Dan Martin (UAE Team Emirates), and in 2015, he lost it to Richie Porte (BMC Racing) by just four seconds.
As for Bernal's ambushing Valverde at an intermediate sprint, the Spaniard said, "We were attentive, but there was a roundabout just before. The bunch braked, I had to brake too, and dropped back, he was attentive and he got his time bonus."
Valverde seemed untroubled by the dent in his GC advantage, though. When asked if he thought those three seconds could be important, he answered simply, "I don't think so. He can win by three seconds or 10 seconds, but I don't think it's going to be significant. Either way, good for him for getting them."
As for the final, unclassified climb prior to the 90kmh descent to Vielha, Valverde ended up putting his house in order with a final, blistering, charge over the summit that put paid to all the previous GC attacks by his rivals.
"They were going for it all over the place," he said. "If it wasn't one then it was another, so I thought, 'Right, it's my turn.' We got into the tunnel and everything calmed down, we just went flat out to try and catch the breakaways."
As for his strategy from here on out, Valverde said, "It's up to me to play a defensive game, and up to Bernal to attack. My situation is very good, and if all goes normally, then I should be OK. But it's a narrow advantage and it can all go up in smoke in any moment: you can crash or end up suffering badly in the cold and get dropped. I'm closer, but it's not over yet."
There was also, one journalist reminded him, the possibility of a last-ditch victory in Barcelona's Montjuic Park, where Valverde has won in 2015 and 2017 and a 10-second time bonus. "That's true," Valverde recognised with a grin, "but in any case, I'm ahead, and as things stand, even if Bernal won on Sunday, I'd still be the winner."