Geraint Thomas has maintained his bid to become the oldest Giro d'Italia champion despite having his lead trimmed slightly by his closest rival Primoz Roglic on the toughest stage of the race.

Slovenian Roglic crossed the summit finish of the so-called "Queen Stage" three seconds ahead of Thomas at the end of the race's final mountain road leg on Friday.

There were no flat sections and five tough, classified climbs on the 183km route from Longarone to the Tre Cime di Lavaredo, which has gradients of up to 18 percent.

The 19th stage was won by Santiago Buitrago, who finished 51 seconds ahead of Derek Gee and one minute 46 seconds clear of Magnus Cort and Roglic, who just missed out on bonus seconds.

But the 37-year-old Ineos Grenadiers rider Thomas remains 26 seconds ahead of Roglic going into what will be a decisive penultimate stage on Saturday.

Third-place Joao Almeida lost more time and was 59 seconds behind Thomas.

Roglic changed his bike shortly before the start of the penultimate climb and he made his move inside the final kilometre.

But Thomas was able to stick to his wheel and the British cyclist made his own attack in the final 500 metres, and briefly looked to have slightly distanced his rival.

But Roglic came back and gained what could yet prove a crucial few seconds.

The winner will likely be decided in Saturday's mountain time trial that ends in a demanding climb up Monte Lussari, with an elevation of over 1000 metres and gradients of up to 22 per cent.

The race ends on Sunday in a mostly ceremonial finish in Rome, where Thomas, who turned 37 on Thursday, could beat the age record held by Fiorenzo Magni, who was 34 when he won in 1955.

"I lost a couple of seconds on the line but it was nice to gain some time on Joao, it will be super close tomorrow," said Thomas, who's seking to add a second grand tour triumph to his palmares after winning the 2018 Tour de France.

"I think it's going to be exciting to watch, horrible to do."

It's far from over. "It's good. We got a bit of legs back, so tomorrow we go full, eh?" said Roglic.

"If I wouldn't be confident then I don't start. The best one at the end wins."

Australian star of the day was Team Jayco AlUla's Michael Hepburn, a rider with no climbing pedigree who surpassed himself to head the field deep into the brutal latter stages before he eventually cracked and ended 17th, 3:13 down.

Jack Haig had another tough afternoon in a fiendishly trying race, the Bahrain Victorious team leader ending up more than 16 minutes behind on the day. He's still Australia's leader in the GC standings, though, although now in 19th at 31:12. Jay Vine is 36th and Rohan Dennis 38th.