The eighth-seeded Williams, who is 851-149 for her career but hadn't played since the Australian Open, came from a break down twice in the opening set and saved the first three set points she faced in the tiebreaker before Podoroska converted her fourth. The Argentine went on to win in 1 hour, 58 minutes.
"It's tough to have a first match on clay. It was definitely kind of good to go the distance and to try to be out there, but clearly I can do legions better," said Williams, who has won the Italian Open four times. "Maybe I do need a few more matches, so I'm going to try to figure that out with my coach and my team and see what we would like to do. ... I have been training for months, but it feels definitely different on clay to make that last adjustment."
Podoroska, 24, took a 5-2 lead in the second set but was broken at love the first time she tried to serve out the match. However, she broke Williams again in the 12th game to seal her third career win over a top-10 player.
"It's special win," Podoroska said. "As I said, she's a great athlete. She did too many things for our sport. It's history. So for me, it's very special. But most of all, I'm happy the way I played. Like, I felt again comfortable on clay, on my game, and that's most important for me."
The third-seeded Halep was leading 6-1, 3-3 before her injury.
In the men's tournament, Rafael Nadal held off a strong challenge from local teenager Jannik Sinner to prevail 7-5, 6-4. Earlier, third-seeded Daniil Medvedev lost 6-2, 6-4 to fellow Russian Aslan Karatsev.