Richie Porte rode into 10th place overall at the Giro d'Italia following one of the most memorable and dramatic Grand Tour stages of recent years.

Stage four always looked likely to be a tough test but no one was prepared for a level of attrition which saw the peloton decimated across 150 kilometres.

Porte hung tough in the hills on a day of constant attacks and tactical twists, and was equal to a late acceleration from Fabio Aru (Astana) on the final climb.

10th on the stage as a group of just 12 riders followed home solo stage victor Davide Formolo (Cannondale-Garmin), Porte now occupies that same position overall, 37 seconds back on the maglia rosa.

Team Sky tried to retain a level of control amid the carnage, placing both Kanstantsin Siutsou and Salvatore Puccio up the road early during a barrage of attacks.

Porte sat in the bunch as they watched a sizable break gain over 10 minutes early on, with the race looking set at one point to flip on its head.

Yet a searing pace from Astana over the final pair of climbs thinned out the group to barely 20 riders. Leopold Konig and Mikel Nieve remained to the fore in support of Porte, who then had Siutsou for help on the run-in as the race came back together.

Simon Clarke (Orica-GreenEdge) was second across the line, leading the group home to pull on the pink jersey after team-mate Michael Matthews fell away, as did most of the peloton, on the Passo del Termine.

"It was by far the hardest stage so far," said Porte after crossing the line.

"I think Astana today were absolutely incredible. Very impressive. I think without them maybe the race was already lost. It's good it feels like the Giro has really started."

Siutsou finished 11th on the stage and also sits 11th overall just behind Porte, while Elia Viviani held on to the red points jersey courtesy of his points haul on day two.

Viviani helped cover moves for Team Sky early, but as the situation solidified a group of 29 riders found themselves a long way up the road.

Big names such as Roman Kreuziger (Tinkoff-Saxo), Franco Pellizotti (Androni-Sidermec), Dario Cataldo (Astana) and Giovanni Visconti (Movistar) added up to a dangerous and race-altering move.

After some brinkmanship from the peloton, Tinkoff-Saxo and Astana upped the pace despite having men up the road. It was the latter who caused the most damage, tearing the race in half on the penultimate third-category climb.

Lack of cohesion in the break and a rampant chase saw their advantage slashed to 1:38 across the line as they began the bell lap in La Spezia. Aru made the first big GC move of the race, attacking on the Biassa climb, drawing an immediate reaction from Alberto Contador (Tinkoff-Saxo), Porte and Jurgen van den Broeck (Lotto Soudal).

By this time Formolo already had a strong advantage, hitting his fellow escapees on the flat run-up to the climb and holding on for his first solo victory.

"It was a pretty busy day out there," said Sports Director Dario Cioni with a hint of understatement back at the team hotel.

"It was actually quite hard to follow what was going on at times as we didn't always have the information quickly from race radio.

"Everyone was getting a bit nervous after that start. Even the sprinters were getting quite nervous with the time cut not being that much today. It could have been a tricky situation for a lot of people.

"I think it was a combination of factors that led to it being so hard. Yesterday and today were really hard stages. A lot of it is also down to the course and the really twisty roads. If it had been on straight and wide roads the selection would have been a lot less. Yesterday's stage definitely took its toll, but everyone still went hard from the word go.

"Tomorrow could go one of two ways. Either everyone is tired from today and takes it a bit easier, or it's going to be another really solid and selective day."