After five easy victories, Monomoy Girl proved she could also handle challenges in the Kentucky Oaks.
The filly first overcame an outside post start before gutting out a close win to maintain its hot start.
Monomoy Girl took charge near the far turn, and the filly outduelled Wonder Gadot down the stretch to win the $1 million Kentucky Oaks by half a length on Friday at Churchill Downs.
The chestnut filly charged from the outside No. 14 post into contention and the lead and held ground in a thrilling side-by-side stretch battle.
Monomoy Girl entered the Oaks with five wins in six starts by a combined margin of 20 lengths.
Her latest triumph was much tighter, which made the signature win before 113,510 all the more satisfying.
"It's great, you know," jockey Florent Geroux said. "She was in a war, she had only one try.
"This filly has been very special for us, known for a long time. We've been pointing to this race since November, to get here in good shape and winning. She was just outstanding."
The filly's win first had to be confirmed by racing stewards.
Wonder Gadot's jockey, John Velazquez, filed a foul claim against Monomoy Girl for a bump down the stretch.
The claim was quickly dismissed, giving trainer Brad Cox a defining hometown win in the 144th running of the Kentucky Oaks.
Monomoy Girl's Oaks win came hours after last year's winner, Abel Tasman, was beaten in her return to Churchill Downs as a four-year-old.
The filly finished fourth among six horses in the Grade One La Troienne Stakes, in which Salty beat Farrell by a head.
Kentucky Derby winner Always Dreaming also had a tough day.
The colt and race favourite in the Grade Two Alysheba Stakes got out fast and battled eventual winner Backyard Heaven through the final turn but faded to a fifth-place finish.