Australian star Ben O'Connor has suffered the first major setback in the leader's red jersey at La Vuelta a Espana.

The man from Perth struggled on the final climb of Saturday's stage eight in Andalucia as race favourite Primoz Roglic attacked for a glorious victory, while reducing the Australian's advantage by 56 seconds.

Decathlon AG2R La Mondiale's O'Connor still leads the race by three minutes 49 seconds, but three-time champion Roglic will have scented weakness and is likley to look for blood in Sunday's critical, mountainous 'Queen' stage.

"Yeah, man, got a bit hot. It was so hot," sighed O'Connor, putting on a brave face after he battled home 17th on Saturday's stage as Red Bull BORA hansgrohe's Roglic powered to his second victory of this year's race 46 seconds ahead, while also picking up a 10-second win bonus.

"I was a bit disappointed how I went at the end because I didn't feel super clutch there.

"I don't think we panicked, I thought as a team we were were great, and actually had everything under control.

"But it's more (time) than I wanted to give away today. It was definitely not my absolute best day, compared to the others. Hopefully, there'll be better days to come."

Spain's Enric Mas (Movistar) was beaten to the line by Roglic at the end of the 159km trek from Ubeda to Cazorla, while compatriot Mikel Landa came in third.

It was the second day running that Roglic had clawed back some of the huge advantage that O'Connor had opened up after his fantastic and unexpected race-blowing solo win on the sixth stage.

The Australian is bound to be put under even more pressure on Sunday's stage from Motril to Granada, which will feature nearly 4500m of climbing including three category 1 slogs, but he sounded cool enough about the forbidding challenge as he knows these ascents well.

"I don't mind these climbs actually in Sierra Nevada, so hopefully I can have a good day and keep this red jersey on my shoulders," said O'Connor, who will then at least have a chance to recover on Monday's rest day before looking forward to the cooler climes in Spain's north-west.

Other luminaries suffered too. Joao Almeida had been third overall at the start of the day but faded badly to tumble in the standings to 26th, over nine minutes down. Mas has moved into third, 4:31 behind O'Connor.