Flavia Pennetta said her fellow Italian Roberta Vinci is capable of ending Serena Williams' Grand Slam attempt.
Williams has a 4-0 head-to-head record on Vinci, the Italian having never previously won more than four games in any one set against the world number one in those meetings.
But Pennetta, who moved into the semi-finals with a three-set win over fifth seed Petra Kvitova, said Vinci's all-round game made her a threat to the 21-time major winner.
Pennetta compared her compatriot to world number 41 Monica Niculescu, a player who caused Williams trouble at Indian Wells earlier this season.
"Roberta, she's completely like a different player," Pennetta told a news conference, ahead of her last-four clash with Simona Halep.
"She's maybe the only one [of her kind]. And there is another one, Niculescu. They play different in that way.
"But I think she has a chance tomorrow. She has to go in the court and try her best. She don't have nothing to lose. She has in front of her Serena Williams.
"You just have to go on court and try everything. She has played a really good tournament.
"She has to believe that she can make it [to the final], because if you go on the court you are not that sure or you are thinking, 'OK, it's done; I'm okay, like this, it's going to be bad'."
Pennetta also fired a shot at US Open organisers, conceding the half-finished Arthur Ashe Stadium roof created a harsh juxtaposition between the shade and the light.
"If I could choose I prefer night session, of course because I like the lights. It's not the big problem for me with playing with lights," she said.
"And playing with this, the shade on the court was a disaster today. I was not able to see.
"But it was the same for both of us."
Pennetta is into the semi-finals at Flushing Meadows for the second time in three years, and will take on Halep in Thursday's night session.