Fifth seed Tsitsipas is chasing his second title of the season after he dismantled Ruud 6-1, 6-4 to win the Monte Carlo Masters last week for the third time in four years.
The two will meet again in the showpiece match on Sunday after Ruud beat Argentina's Tomas Etcheverry (24) 7-6, 6-4 in the other semi-final earlier on Saturday.
Lajovic broke to take the opening set against the Greek after neither player dropped serve in the first 11 games.
Tsitsipas committed 22 unforced errors in that set but his performance improved and he broke Lajovic's serve in the second to take a 4-3 lead and eventually level the match.
Another two breaks in the decider put Tsitsipas 4-1 in front and he marched on to book his spot in a fourth Barcelona final.
"I'm sure having played someone a few days apart, for sure they're going to come up with new plans and new adjustments," Tsitsipas said when asked about his rematch with Ruud.
"I'm expecting a much better version of what I had in Monte Carlo... Of course these kind of matches are tricky, playing back to back. You try to see what works best for you. The opponent is trying to get as good as he can for the next one."
