Madrid - Little-known American Sepp Kuss powered to a landmark summit finish win for Jumbo-Visma at the Vuelta a Espana on Sunday as his captain Primoz Roglic battled over a series of mountains to retain the overall lead.

The 15th-stage win earmarks Kuss for a strong future in Grand Tour riding.

Katusha's Ruben Guerreiro came second on the day as up-and-coming Ineos star Tao Geoghegan Hart of Britain was third.

In the battle for supremacy in the overall, the pretenders to the title ganged up on Roglic to test his endurance on a difficult day.

But the Slovenian appeared to work with second-placed Alejandro Valverde as the pair ascended the last climb together, damaging their top-10 rivals and even shaking hands after the finish line.

"It was a nice day for us, but there are many hard days to come," said Roglic.

"He (Kuss) was waiting for me but he had the chance and we told him to take it."

They put 29 seconds into Tadej Pogacar, Miguel Angel Lopez and Rafal Majka and a further 25 seconds into former champion Nairo Quintana.

Roglic emerged the big winner of the pair, however, retaining a 2:25 lead on Valverde with three more major climb stages from the six race days remaining on the road to Madrid.

But Stage 15 was also an amazing day for the fast-rising Jumbo-Visma team with the emergence of the 24-year-old Kuss, who high-fived dozens of roadside fans on an exultant final 500m after he scampered clear halfway up the steep ramps to the Santuario del Acebo summit finish.

"It was crazy, all those high-fives. The road was a bit of a goat track in parts on that last climb. It hasn't sunk in (the win) and I'm lost for words but this is what makes bike racing so special," Kuss said.

The American skipped up the shoelace turns of the gnarled pathway even when the gradient was nudging 20 percent.

Jumbo-Visma won four stages on the Tour de France in July with Steven Kruijswijk ending third after Roglic also placed third on the Giro d'Italia in May.

Barring a fall, Roglic looks a safe bet to win his first Grand Tour here in Spain with the team due to unleash their superstar signing, former world champion and Giro winner Tom Dumoulin, in the new season.

The peloton embarked from the former gold mining town of Tineo on a cool morning for a fast-paced 154km slog over four mountains including the terrifyingly named Puerto del Pozo de las Mujeres Muertas (mountain of the well of dead women).

Monday marks the third chapter of the Vuelta's tour through the Cantabrian Mountains with two category-one climbs ahead of another 'hors-category' ascent to Alto de La Cubilla.

Vuelta a Espana 15th stage results on Sunday:

Stage results

1. Sepp Kuss (USA/JUM) 4hr 19min 04sec, 2. Ruben Guerreiro (POR) at 39 seconds, 3. Tao Geoghegan Hart (GBR/INEOS) 40, 4. Oscar Rodriguez (ESP/) 53, 5. Mark Padun (UKR) 1:49, 6. Ben O'Connor (AUS) 2:05, 7. Lawson Craddock (USA) 2:11, 8. Primoz Roglic (SLO) 2:14, 9. Alejandro Valverde (ESP) 2:14, 10. Sander Armee (BEL) 2:48

General classification

1. Primoz Roglic (SLO/JUM) 58hrs 10min 32sec, 2. Alejandro Valverde (ESP/MOV) at 2min 25 sec, 3. Tadej Pogacar (SLO/UAE) 3:42, 4. Miguel Angel Lopez (COL/AST) 3:59, 5. Nairo Quintana (COL/MOV) 5:09, 6. Rafal Majka (POL/BOR) 7:14, 7. Nicolas Edet (FRA/COF) 9:08, 8. Wilco Kelderman (NED/SUN) 9:15, 9. Carl Fredrik Hagen (NOR/LOT) 9:44, 10. Hermann Pernsteiner (AUT/BAH) 11:39.