Arnaldi: 'Best match of my life' to upset Ruud in Madrid
Fifth seed Andrey Rublev continued his hot form on clay as he beat Stanislas Wawrinka 7-5, 6-4.
Carlos Alcaraz begins the defence of his title later in the day against Emil Ruusuvuori while women's number one Iga Swiatek faces Julia Grabher.
Arnaldi, a 22-year-old Italian ranked 105 in the world, who saved a match point against Benoit Paire in the first round, hit 35 winners on his way to a more comfortable victory over Norwegian Ruud, who reached two Grand Slam finals last year.
"The ball bounces so high and I didn't like it at first. I struggled a bit," Arnaldi said on court at the end. "But today, I don't know, maybe the stadium, maybe the pressure on him, but I played the best match of my life."
The Italian broke serve twice in the first set to take a 5-3 lead and close the set on his serve.
He broke again in the first game of the second set and held his serve the rest of the way.
In the third round, Arnaldi will face Spaniard Jaume Munar who advanced when Dutchman Tallon Griekspoor retired with an injured ankle in the second set.
Rublev started the clay season by winning in Monte Carlo and then reaching the final in Banja Luka, where he lost to Serbian Dusan Lajovic.
He needed just 1 hour and 17 minutes to dispatch Wawrinka, the 38-year-old Swiss triple major winner.
Rublev broke in the 11th game of the first set. He fell behind to an early break in the second before winning five straight games and then serving out for the match.
"When I saw the draw I thought that it was amazing being the fifth seed and having Wawrinka in the opening round," Rublev said.
"When I was World No. 40, I sometimes had an easier draw. I was thinking 'OK, maybe I would go home early'. In the last meeting, he beat me, so I was thinking to focus and see what would happen but I was able to win today and I feel great."
Rublev will face Japan's Yoshihito Nishioka, who beat Slovakian Alex Molcan 5-7, 6-4, 6-4.
Another seed to fall was number 15 Italian Lorenzo Musetti, who lost to Germany's Yannick Hanfmann 6-4, 7-6 (7/3).
