The detectives flew to the Algarve, where the barmaid lives, and questioned her about her relationship with the pedophile, who they said abducted and murdered the little girl in May 2007.
She is still too afraid of her ex to reveal his identity because he would have terrorized her while they were in a relationship and she lives in fear that he will one day be released.
But in a major escalation to their investigation, officers reassured her that Brueckner would be behind bars for a long time.
A friend of his ex told the Mirror: “They told him not to worry about his release.
“They said he was going to stay inside for a long, long time. She sat with them for 12 hours. It was exhausting and overwhelming for her. She was really upset but they reassured her that she was an important character witness.
“They wanted her to go to Germany and meet them and said they would pay for the flights and a hotel.
“But she didn’t want to leave, so two detectives flew to Portugal to meet her instead.
“They wanted to know everything she knew about Christian and everything he did to her. They said every little detail about him was helpful in helping analyze patterns of behavior.
Last month’s interview with the waitress was the first time German police were allowed to work on the case in Portugal. Three Portuguese officers and two translators also participated in the interview in Faro. The Portuguese police have already questioned her.
And former colleagues at the Lagos bar where she worked with Brueckner were also questioned by police. German prosecutors are desperate to build a case against Brueckner, 43.
They claim to have proof that Madeleine is dead but do not have enough evidence to charge their suspect at this time.
The Mirror revealed in June how he allegedly tormented his Berkshire ex on several occasions when they met in 2004 and 2005. He allegedly broke into her apartment through a window and watched her under her bed.
The alleged incident took place hours after he brutally assaulted her in a jealous rage because she had innocently hugged another man.
Brueckner reportedly repeatedly banged her head against a wall in the ladies restroom at the Algarve bar where she worked – leaving her in a bloody mess.
When she returned home, she had a “horrible feeling” that someone was inside and found her hiding. She thinks he was waiting to attack her if she brought back another man. Instead, the rapist, then in his twenties, stared at her and said “goodbye” to her before running away.
Madeleine was three when she disappeared from her family vacation apartment in Praia da Luz while parents Kate and Gerry were dining with friends at a nearby tapas bar.
New main suspect of Madeleine McCann
In June, Brueckner, who was living in Praia da Luz at the time, was named a suspect in the case by German authorities, but he denies any involvement. He is serving a 21-month sentence for drug-related offenses in Germany.
Brueckner was convicted last year for raping a retiree at the resort in 2005.
He is expected to begin a seven-year sentence once his appeal process is exhausted. There were fears that he could be released on legal grounds, but a lawyer for the European Court of Justice advised him to stay behind bars.