Championship leader Manuel Gonzalez completed a dominant performance to take a vital Moto2 victory at Mugello.
The Dynavolt IntactGP rider recovered from eighth on the grid to win the race by 1.409s, as he extended his championship advantage to nine points.
Polesitter Diogo Moreira made a good start and held the lead through the first corner ahead of Aragon race winner Deniz Oncu.
Aron Canet pulled a move on championship rival Gonzalez to go third, as the pair began tussling for the first time on track.
Heading down the main straight for the first time, Oncu took advantage of the slipstream and overtook Moreira for the lead.
The leading group was made up of five riders as they headed down the start/finish straight for the second time, with Moreira regaining the lead and Arenas taking second from Oncu.
Arenas had the confidence and made a move on Moreira to lead of the pack over the line for the fourth tour of the circuit.
As the riders continued fighting for the podium positions, the group expanded to a nine-rider field, with Canet taking the initiative to lead the race.
The lead continued changing as the laps counted down, with Arenas returning to the lead as rivals Canet and Gonzalez recommenced their battle.
Filip Salac and Izan Guevara had the momentum and used their advantage to overtake Oncu for fifth and sixth.
As these battles continued, Gonzalez decided it was time to strike as he overtook compatriot Arenas for the lead of the race.
The championship leader would not remain in control for long, however, as Arenas used the slipstream effect down the main straight to take the lead once more.
The pair would continue their battle for the lead, as both riders used the slipstream to full effect in an attempt to take control of the race, though Gonzalez was able to guide the leading four away from the rest of the group.
Gonzalez had something extra as the tyres started to degrade as he pulled over a second clear of Arenas, as Canet and Moreira continued battling for the final podium position.
The battle for third continued, as Canet launched a move on the Brazilian, who knew the Spaniard would run wide and allowed him to sweep under and maintain the position.
On the penultimate lap, Moreira thought he had seen off the challenge from Canet but a squirm on the exit of Turn 1 allowed the Spanish rider to take third.
The battle for third would continue onto the final lap, which brought Celestino Vietti into contention as the pair scrapped throughout the last tour of the Mugello circuit.
Out front, Gonzalez crossed the line to complete his comeback from eighth on the grid as he took victory by over a second.
Arenas took the flag in second place, as Canet prevailed in the battle for the final podium position in third.
Moreira was forced to settle for fourth, with the fast-charging Vietti finishing a tenth behind in fifth.
Oncu took sixth ahead of Guevara in seventh, with David Alonso battling through the order to eighth.
The American Racing pair completed the top 10, with Joe Roberts finishing ahead of team-mate Marcos Ramirez.
Barry Baltus crossed the line in 11th ahead of Salac, whose degrading tyres saw him take 12th.
Senna Agius was 13th after defeating 14th-placed Alonso Lopez in the run to the line.
Dani Holgado rounded out the points as he finished 15th.
