Italy's top player came out wearing a black shirt with a red lightning bolt design down his chest and he was energized from the start, grazing the flower beds lining the court to return Thiem's high-bouncing topspin serves and frustrating the Austrian with well-placed lobs and touch volleys.
"I played very well today," said Thiem, who beat Rafael Nadal in the Madrid Open semifinals last week. "It was just him, he was very pumped playing at home."
Nadal, a seven-time Rome champion, and four-time champion Novak Djokovic also advanced. Nadal routed 31st-ranked Damir Dzumhur 6-1, 6-0, while Djokovic beat Georgian qualifier Nikoloz Basilashvili 6-4, 6-2.
"It was important after a loss in Madrid to come back strong. And that's what I did today," Nadal said.
While Fognini drop-kicked his racket onto the court after a miss early in the third set, it was Thiem who ultimately lost his cool. In the final game, Thiem smashed his racket into a mangled wreck and handed it to a befuddled fan in the front row.
When Thiem netted a service return two points later to end it, the White Stripes' Seven Nation Army was blasted over the stadium sound system and the crowd began singing along in an atmosphere that more closely resembled a soccer game than a tennis match.
"We're finally connecting and I think that's the most beautiful thing," Fognini said of the crowd, which whistled him off the court following an erratic performance a few years ago. "They finally understand me.
"I beat a top clay-courter," Fognini added. "I think I deserved it at the end. ... I ruined a few flowers but I think the federation can afford to buy some new ones."
Thiem was already looking ahead to the French Open, which starts on May 27.
"It's a loss which is not too painful, which is just motivating me for the next weeks," the Austrian said.
Last year in Rome, Halep rolled her ankle in the final and lost a lead and the championship to Elina Svitolina.
At 5-5 in the third, Pliskova got engaged in a lengthy argument after she had an overhead smash called out. The chair umpire acknowledged that she "lost" the ball mark and therefore said the point should be awarded to Sakkari. On the next point, Sakkari converted a break then served it out.