Matteo Berrettini put Italy into the city final of the inaugural United Cup with a brilliant attacking win over world number three Casper Ruud in Brisbane on Tuesday, while Maria Sakkari helped send Greece through.

Berrettini broke Ruud once in each set for a clinical 6-4, 6-4 triumph that gave Italy an unassailable 3-0 lead over Norway.

Italy will play either Poland or Switzerland in the Brisbane final, with the victor of that tie to be decided later Tuesday.

The winner of each city final in Sydney, Brisbane and Perth will play the semi-finals in Sydney, along with the best-performing team from the beaten finalists.

Ruud, who had a brilliant 2022, went into the match as the slight favourite, but Berrettini was superb from the outset.

He served well throughout and his powerful groundstrokes put the Norwegian under enormous pressure.

Berrettini hit 10 aces and eight forehand winners, with the power and placement of his shot-making causing big problems.

Berrettini, who has slipped to 16th in the world, said he knew he had to serve well against Ruud.

"He's improved so much in the last year so I knew that I had to serve my best," he said.

"We know each other pretty well so I knew I had to serve like that, but it's one thing knowing I had to and one thing doing it."

Lucia Bronzetti later made it 4-0 to the Italians with a 6-2, 7-5 win over Ulrikke Eikeri.

In Perth, world number six Sakkari ensured Greece moved through to the final when she downed Belgium's Elise Mertens 6-1, 7-5.

The Greeks went into the match needing to either win the tie or lose 3-2 to progress to the city final against either Croatia or France.

Sakkari's point made it 2-1 to Greece, cementing their finals position.

"I knew the match would be critical, so I was nervous, especially against a world-class player like Elise," Sakkari said.

"When you play a regular tournament and you lose, it's just for yourself, but I wanted to win today for the team more than myself.

"It means a lot to play for your country."