Fans were treated to a roller-coaster encounter on Centre Court as both players exhausted themselves seeking a spot in the final against either Barbora Krejcikova or 2022 champion Elena Rybakina.
The pair were in unchartered territory, with Vekic playing the first major semi-final of her career while Paolini had never even won a match on grass before this year.
The Croatian took the opening set comfortably having broken Paolini in the fifth game and then again in the seventh with her mixture of groundstrokes and drop-shots proving too much for the Italian under a sunny sky.
However, Paolini, cheered on by a loud contingent of Italians in the crowd, became more aggressive in the second as she approached the net more and put Vekic on the backfoot. Her diligence and energy eventually paid off with a break to clinch the set.
The pair exchanged breaks in the decider as both racked up the unforced errors, particularly Vekic who looked fatigued and emotional as the match wore on. She was in tears after saving Paolini's first match point.
A successful challenge overturning a line call helped Paolini hold an almost 10-minute game that ended in Vekic sobbing in her chair at the changeover.
The Croatian rallied to save another match point and held serve to force a tiebreak but looked particularly laboured as she made 57 unforced errors in total and voiced her frustration to coach Pam Shriver in the players' box.
Paolini, in contrast, looked just as energised as she did at the start. She prevailed and won the two-hour, 51-minute epic, during which both players ran more than 3.5 kilometres, to reach her second successive Grand Slam final.
"These last months have been crazy for me. I am trying to focus on what I have to do on court and I love playing tennis. It is amazing to be here and it is a dream," Paolini, who reached the semi-finals in Eastbourne last month, said on court.
"I think it was an intense match and I tried to play my best and now it's time to recover. I think I need an ice bath as my legs are a little bit tired."
The previous longest women's semi-final at Wimbledon was two hours and 50 minutes between Serena Williams and Elena Dementieva in 2009.
