He both consolidated his third place overall and boosted his chances for overall victory on Sunday by taking fourth place, just 25 seconds short of stage 13 winner and new race leader Primož Roglič (Jumbo-Visma).
The British racer rated the stage as his best-ever time trial performance, in which he notably put 24 seconds into the now second-placed overall Richard Carapaz (Ineos Grenadiers).
Barring major surprises, the Briton looks on track for at least a third place in Madrid after gaining ample time on Dan Martin (Israel Start-Up Nation), previously three seconds behind in fourth place, who is now 55 seconds behind the EF Pro Cycling rider. Spain's top hope, Enric Mas (Movistar), remains in fifth place but is 2:36 behind Carthy and only a minor threat to his podium position.
Carthy's time trial performance, he told reporters afterwards, had begun very strongly, and the split times bore that out, placing the Briton two seconds ahead of Roglič at the first time check after 12 kilometres, and four seconds up on Carapaz. At the second check after 24km, Roglič had moved ahead by a second and would open up a much bigger gap on Carthy on the final climb. But Carapaz was more than 18 seconds down by the second check and both Martin and Mas were even further adrift.
"It went very well," Carthy told reporters, "I could recover after each little climb on the flatter part, and the bike change" - at the foot of the Ezaro and always a potential trouble point of any time trial - "went perfectly, even better than the three or four times we practised this morning."
Carthy's strong performance in the opening kilometres as he went head to head with Roglič had Spanish TV commentators nearly in a state of shock early on, but there was evidence aplenty in the past that the Angliru winner was by no means as poor against the clock as some other expert climbers - both from his times as a junior and young racer through to the eighth place last year in the Giro d'Italia in the San Marino TT. But if there was ever a need for confirmation that Carthy is an allrounder for GC rather than only being able to gain time in the mountains, it was on the roads of Galicia that the 26-year-old showed it most clearly to date in his career.
"It came out well and I'm happy with what I've done, but there are four more important days to come," he told a fired-up Spanish radio reporter about what remained in the Vuelta in its third week. "Do you want more?" the reporter asked him point-blank to which Carthy answered simply "Si" [Yes]. "Do you want the Vuelta?" "Supongo que si - I guess so" came back his laconic, but equally convinced, answer.
Carthy promised another reporter that he would go on fighting and the race's last mountain top finish of the ski station of La Covatilla - where, as it happens, another Briton, Simon Yates (Mitchelton-Scott) took la roja when the Vuelta reached there last, back in 2018 - is likely to be the most obvious set piece day for a GC battle.
However, Wednesday and Thursday's long grinds across Galicia are very treacherously tough terrain, with several thousand metres of vertical climbing on Thursday in particular and Friday's trek through the little-known Sierras de Francia in western Extremadura could also produce some surprises.
"I'm going to go on fighting til Madrid," Carthy promised, and with both his head and legs clearly in good shape, as he said, the tide in the third week of the Vuelta is certainly flowing in his favour right now.
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