Tomic accounted for Seppi in 71 minutes before 16-year-old Ashleigh Barty toppled former French Open champion Francesca Schiavone 6-0 6-3 to give Australia a 2-0 win over the Italians.
"I played really well against Novak yesterday night and today was really good as well," Tomic said.
"It's amazing preparation before the Australian Open and I'm really glad I came. The last few months were very hard. I was focusing a lot on my fitness and my serve. You need to serve well when you play against these top players."
Australia will reach Saturday's final if Germany upset Serbia's top-seeded pairing of Djokovic and Ana Ivanovic on Friday.
Tomic's three successive wins against Tommy Haas, Djokovic and Seppi, complemented by Barty's success against former French Open champion Schiavone, have raised expectations of home success at the Australian Open starting on January 14.