The Team Sky rider came fifth in stage 15's mountain finish in the Sierra Nevada after a triple whammy of climbs, 47 seconds behind Colombian stage victor Miguel Angel Lopez.
A late surge by Froome over the final meters of the punishing ride over two category one and a final special-category ascent allowed him to increase the gap over Vincenzo Nibali from 55 seconds to 1:01.
Alberto Contador attempted a twin attack with Nibali in the final 3km to dart 10 seconds ahead of the red jersey group, but could not last and finished six seconds behind Froome, in seventh.
Russia's Ilnur Zakarin moved up to third overall by finishing second behind Lopez, 36 seconds off the pace, with Wilco Kelderman third on the 129.4km ride from Alcala la Real. "I kept calm until the last climb and there I wanted to see how the others were doing, and when I saw I had good legs I tried to go alone".
Trentin's hat trick of stage wins gave his Quick-Step Floors team its fifth stage win.
Saturday's stage was the first of two back-to-back summit finishes which will have a big say in whether Froome succeeds in becoming the first rider in 39 years to win the Tour de France and La Vuelta in the same year.
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Froome, who is bidding to become only the third rider to win La Vuelta and the Tour de France in the same year, crossed the line 31 seconds down on the Polish rider after the 175km stage.
"It was a tough day, it was really windy and some teams were trying to split it", continued the race leader.
"I'm just grateful to be in this position".
The three-time Tour de France victor is looking to add his name to the cycling history books by becoming over the third person to win the France and Spain challenges in the same year; if he's successful, it will be the first time since 1978.
The riders of the 2017 Vuelta will now enjoy their second rest day before the race continues on Tuesday. After my first victory I was keeping fresh to try again today.