Nazeef took victory in the Falmouth Stakes at Newmarket as her stablemate Terebellum finished third.
John Gosden-trained Terebellum, ridden by Frankie Dettori, was the 5-6 favourite and led for 10 furlongs, but was overtaken in the closing stages.
Nazeef has won on all six of her starts since finishing third last June on her first run at Newmarket's July Course.
"The only pace in the race really was Frankie's filly," Nazeef's jockey Jim Crowley said.
Nazeef took victory by a neck from Billesdon Brook in second, as both reeled in Terebellum, who had led entering the final furlong.
Crowley said of Nazeef: "She'd won over 10 furlongs and all the others looked like they were going to take a lead. I had a great position in a small field and managed to get a bit of cover.
"She's as tough as nails and she's improved with every run. That's six races in a row she's won now - she's climbed her way up the ladder. "