Gianfranco Zola curled in a free kick from the edge of the penalty area to give Cagliari the lead in the 50th minute at Sant'Elia stadium.
Obafemi Martins equalized, scoring from one meter during a goalmouth scramble off a corner kick.
It was the 21st goal for the Nigerian forward in Serie A and Cup matches this season.
Inter, the favorite among the last four in the competition, started Adriano and Martins in front and replaced them during the second half with Christian Vieri and Julio Ricardo Cruz.
Inter goalkeeper Fabian Carini had some uncertain moments, including a handing outside the area that resulted in Zola's free kick.
Thursday's draw extended Cagliari unbeaten streak at home to seven months.
Inter's coach Roberto Mancini said the away goal may help his team qualification for the final, barring an upset in the second leg at San Siro next Wednesday.
In another semifinal round Wednesday, AS Roma and Udinese played to 1-1 at Stadio Olimpico with a first-half goal by Philippe Mexes and a late equalizer by Udinese captain Valerio Bertotto.
Udinese will host the second leg on Tuesday.
FC Copenhagen reached the Royal League final when Malmo FF beat Rosenborg 4-2 on Thursday, sending the Danish club to the May 26 championship match against IFK Goteborg.
Brazilian Afonso Alves scored in the 22nd and 62nd minutes to twice give Malmo the lead.
Frode Johnsen equalized twice before Markus Rosenberg scored the eventual winner in the 73rd and Marcus Pode added the insurance in injury time.
The result finished Copenhagen atop the group with seven points, one more than Malmo and three better than Rosenborg.
In Thursday's other game, Kalle Svensson scored to give Goteborg a 1-0 victory at SK Brann. Goteborg had clinched its place in the final.
The inaugural Royal League, so named because Sweden, Denmark and Norway are constitutional monarchies, started late last year with the top four teams from the three Scandinavian countries. It's an attempt to shorten the long winter breaks clubs in those countries must endure, and make them more competitive in international play.
Manchester United manager Alex Ferguson was charged with improper conduct Thursday by the Football Association over comments he made about penalties.
Angry at the referee's failure to award a penalty for a foul on Alan Smith a 2-1 Premier League win over Newcastle on April 24, Ferguson suggested there was a conspiracy against his team.
"It is getting ridiculous now. I am not sure if they are instructed, but it is looking sinister to me," Ferguson said the day after the game.
"Since that penalty kick against Arsenal, it seems to me we are not going to get another one, no matter what the circumstances. One of our players will need to get shot for us to get one."
He has until May 26 to respond to the charge.
Manchester United has been awarded two penalties in 27 Premier League games since it beat Arsenal 2-0 in October, snapping the Gunners' 49-match unbeaten streak in the competition.
With the score 0-0, Manchester United was awarded a penalty for a foul by Sol Campbell on Wayne Rooney. Campbell berated his England teammate for diving and TV replays showed minimal contact.
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