Italian Matteo Trentin won stage 10. Four-time Tour de France champion Froome remains 36 seconds ahead of his closest rivals, but did see Roche join Esteban Chaves in joint second place after the Irishman gained 29 seconds.
Ascending the Collado Bermejo, Trentin and Jose Joaquin Rojas formed a breakaway before the former pulled away to win his second stage. Roche had a “few scares” on the 164.8km stage from Caravaca to Alhama de Murcia.
The descent conditions were tricky, with Cofidis rider Luis Angel Mate crashing, but Roche took advantage to skilfully move up the rankings, coming in 29 seconds ahead of the main field. “It was a good day,” he told Europsport.
“It wasn’t really a plan to go on the descent until I saw Vincenzo [Nibali] open up a small gap, but I was there with him and said: ‘Let’s go and see what happens.’”
Froome said he wasn’t prepared to gamble on yesterday’s stage: “For me, I wasn’t willing to take any risks. I am in a good position in the GC and there was a lot of roads from the bottom of the climb to the finish,” the Team Sky leader said.
“We took it easy with my team-mates and kept everything together. Nico Roche wasn’t a big worry for me.
“As I said, I was happy to stay safe, but he obviously did a great ride. He’s in fantastic shape in La Vuelta, so congratulations to him.”
Roche told Cyclingnews that the attack had been anything but planned and that unlike many riders who have done the Collado Bermejo in the Vuelta a Murcia one-day race he was on completely new terrain making the descent.
“I’ve never been here,” Roche said afterwards. “I just said ‘let’s go.’ It was a proper improvisation. I said to myself, I’m not the worst descender in the peloton so it could be a real opportunity particularly as there are a few splits amongst the group. So I thought I’d give everything.”
The descent was as dodgy as it looked on TV, he said. “I had a few scares. I locked up my back wheel trying to go on the inside of Froomey on one of the hairpins and almost came down and that happened to me a few times on the descent.
“But it’s part of the job, I’m not a big fan of those kinds of descents for finishes, but today it definitely suited me.”
The second week has got off to a good start for Roche, even though he recognised it will be a different situation on stage 11’s summit finish at Calar Alto.
“It’s going to be very different to today’s finish. I’m definitely in great condition, but Wednesday is a big test with the high mountains and let’s see how it goes.”