Casper Ruud said having the world number one ranking on the line is providing "extra motivation" after he reached a first US Open semi-final with a scintillating victory over Matteo Berrettini.

Victory lifts the 23-year-old Norwegian to number two in the world rankings.

Ruud has the chance to take the top spot if he wins his semi-final against either Nick Kyrgios or Karen Khachanov.

The pair face each other later on Tuesday.

"I had no idea I could be number one when I started this tournament," said Ruud, who was runner-up in this year's French Open. "It is something all young players think about.

"It's a bit of extra motivation to dig in and keep fighting. If I am very lucky I can leave New York as world number one.

"[But] I don't want to think too much about it."

Ruud must make the final to have a chance of pipping Rafael Nadal to top spot in the rankings. If the Norwegian was to meet Spain's Carlos Alcaraz for the trophy, the winner would become world number one.

Under the roof Ruud got off to an electric start, taking the first set with ease and going up 5-1 in the second.

But, after an hour of one-way traffic, 2019 semi-finalist Berrettini fought back to 4-5 before Rudd scraped through to lead by two sets.

The Italian, playing in his fifth consecutive Grand Slam quarter-final, broke early in the third set but failed to convert two set points at 5-2.

And Ruud broke back before forcing a tie-break, which he won in composed style to reach his second Grand Slam semi-final.

Berrettini, who has been hampered by injury and illness this season, was at a loss to explain his poor start to the match.

"I don't know what happened," he said. "He played a really good match, and I played a really bad match.

"Really nothing I can say more than [it was] the worst day of the tournament probably in the most important moment. I wasn't feeling my game. I wasn't feeling my mindset. I think he was feeling really good."

Ruud 'ready for anything' against Kyrgios

Australian Wimbledon finalist Kyrgios, who faces Russian 27th seed Khachanov in the second match of Tuesday's night session, is the favourite to meet Ruud in the last four.

Ruud and Kyrgios have history - Kyrgios was defaulted from an Italian Open match against the Norwegian in 2019 for throwing a chair across the court.

After the match Ruud called for the Australian to be banned for six months and called him an "idiot".

Kyrgios later went on to describe Ruud's tennis as "boring".

After his victory over Berrettini, Ruud was asked about the prospect of potentially facing Kyrgios, replying: "Let's see if it will be some drama or not, but I'm ready for anything."

Kyrgios beat Ruud at Indian Wells earlier this year in straight sets.

"I think Nick [Kyrgios] has taken steps as a player," Ruud added. "Obviously, he has improved and he will be a dangerous player for sure."