Great Britain's Cameron Norrie and Dan Evans both secured progression into the last 16 of the Barcelona Open with victories in their respective second-round contests on Wednesday.
Norrie exploded out of the blocks against Kotov and quickly earned the double break during a five-game winning streak, but he was forced to fend off four break points for the Russian in the fifth game.
The British number one subsequently squandered two chances for a first-set bagel before gleaning a 1-0 lead with a hold to love, and the start to the second set followed a similar pattern.
Norrie extinguished any hopes of a Kotov fightback, and two late holds for the world number 108 just delayed the inevitable, as the Wimbledon semi-finalist cruised through.
Following Norrie's success, Evans overcame a couple of scares to send 22-year-old Arnaldi packing, saving two break points in the fourth game before breaking himself in the fifth on an Arnaldi double fault.
The Italian brought up a total of six break points in Evans's three remaining service games, but the British number two saved all of them to take the opening set in just over an hour.
Evans subsequently broke in the first game of the second set to establish a healthy advantage, but as was the case in the opening set, the 32-year-old presented Arnaldi with a couple of opportunities to restore parity.
The Italian failed to capitalise, though, and his double fault in the ninth game brought up two match points for Evans, who converted his first to sail through to the third round in one hour and 48 minutes.
Elsewhere on Wednesday, there were victories for Stefanos Tsitsipas, Jannik Sinner and Denis Shapovalov, while Novak Djokovic reached the last eight of the Srpska open with a three-set win over French teenager Luca Van Assche.