But trainer John Gosden was adamant the win was no fluke.
"I didn't bring her here for the Sussex air," he said after his homebred Dansili filly posted a neck victory over 33-1 shot Thistle Bird, with Hot Snap coming in two lengths back in third in the 1 1/4-mile race.
Ridden by William Buick, Winsili was timed in 2:06.19 to earn her first group victory after finishing fourth in her group debut in the 1 1/2-mile Ribblesdale Stakes (Eng-II) last out at Royal Ascot.
A trip to the $2 million Breeders' Cup Filly & Mare Turf (gr. IT) in November at Santa Anita Park could be in the cards for Winsili.
"She will be running in nice mile and a quarter races and I will probably keep her with fillies," Gosden said. "She will make a lovely broodmare one day and could be a possibility for the Breeders' Cup, but she would have to get there a week in advance and be schooled in the paddock at Santa Anita. I might leave that decision to the owner, Prince Khalid."
QIPCO One Thousand Guineas (Eng-I) winner Sky Lantern, the 7-4 Nassau favorite, was unlucky for a second straight race, finishing fifth in the 14-horse field in her first try beyond a mile. She entered off a heartbreaking runner-up finish in the Etihad Airways Falmouth Stakes (Eng-I) July 12 at Newmarket, in which she battled winner Elusive Kate throughout but finished a neck behind in second when that rival drifted left and carried her out.
Irish One Thousand Guineas (Ire-I) winner Just The Judge did not handle the ground and tore off a shoe before finishing 12th in the Nassau.
With Winsili relaxing toward the back of the field and Sky Lantern racing in midfield in the Nassau, Seamus Heffernan guided Just Pretending to the early lead. In a dramatic moment, the rider nearly took the wrong course midway through race. But he recovered in time and continued with his mount on the front end until headed by Sajjhaa a furlong out.
As jockey Richard Hughes edged out Sky Lantern to go around Sajjhaa, Winsili charged past in the final furlong to challenge the leader, bumping with that rival and hampering Sky Lantern. From there the favorite was boxed in. Sajjhaa finished fourth, a half-length behind Hot Snap.
"I had a good run through the whole way," Hughes said. "I was behind the Godolphin runner (Sajjhaa) at the two-pole and I edged out to go round her. Mr. Buick's horse rolled down on top of me and just pushed me back in on the heels.
"I think that my filly was the best in the race…there was nothing that I could have done. That's racing, but it was very unlucky."
Winsili's victory marked consecutive Nassau wins for Gosden following The Fugue last year. Juddmonte Farms owner Khalid Abdullah enjoyed a fourth Nassau win in the past five years, including three consecutive with Midday in 2009, 2010, and 2011.
"Obviously, she didn't stay a mile and a half in the Ribblesdale at Royal Ascot," Buick said after Winsili's victory. "She's not easy and the whole team at home have done a great job because she is a bit lively in the mornings.
"My game plan was to follow 'Hughsie', and I was hoping that he wasn't going to get too far back, which he didn't. Then I had plenty of room in the straight to maneuver. She traveled so good that I decided to take my time because I know that she has a turn of foot. She fought off all challengers up to the line."
Winsili has three wins and a second from five career starts. A convincing winner in her career debut last August at Newmarket, she tallied her first stakes win in her 2013 bow in May in the Swettenham Stud Fillies' Trial Stakes.
Winsili is the sixth group winner this year and third at the top level for Juddmonte resident Dansili, a 17-year-old son of Danehill.
She is the first starter out of the Juddmonte's winning Pivotal mare Winter Sunrise, a half sister to French group II winner Ice Blue. French group-placed second dam Winter Solstice is a half sister to 2004 Dubai Sheema Classic (UAE-I) winner Polish Summer and 2004 Manhattan Handicap (gr. IT) winner Meteor Storm.