Three miles in testing conditions faced the eight runners in the feature event of the afternoon and Allied Victory cut out much of the running before Mark Enright bustled Miss Xian to the front rounding the turn for home.
Sonny B travelled ominously well into the straight and joined Miss Xian on the run to the final fence, with Bothair Clei also in contention to make it a three-way battle on the run-in.
Bothair Clei looked set to triumph after hitting the front, but 9-2 chance Miss Xian roared back and the two passed the post virtually as one.
After a brief wait, the judge called Flood’s mare the winner by a nose.
Flood said: “That’s worth a lot. It’s a great race, the Leinster National.
“We took a big chance being 6lb out of the handicap, but that’s her third win around Naas and she’s tough and she stays.
“It’s a black type race and it’s worth a lot to a mare, so it’s worth taking a chance.
“I suppose the three-mile-five (furlong) in Punchestown would be a race that springs to mind. We’ll see what the handicapper does to her.”
The long-absent Tennis Cap proved a bitter disappointment as Rossvoss skipped clear in the Hospitality At Naas Racecourse Beginners Chase.
Runner-up in the County Hurdle at Cheltenham two years ago, Tennis Cap had been sidelined by injury since falling at Punchestown at the end of that season, but appeared to have been found an excellent opportunity to make a winning return by Willie Mullins.
With Ruby Walsh and Paul Townend resting up ahead of the Festival, David Casey was in the saddle aboard the 4-7 favourite, but he weakened quickly from the home turn and finished well-beaten in third.
Rossvoss was a 14-1 shot for trainer Ted Walsh and jockey Sean McDermott and readily picked off Kick On Boss on the run to the final fence, before pulling away for a 15-length victory.
Walsh said: “That was a surprise. He jumped well and travelled well but Tennis Cap and Kick On Boss didn’t run their race.
“He won twice at Galway and I thought he’d be a nice horse but he cut a leg at Bellewstown and was off for six or eight months.
“Hopefully he’ll go on to fulfil the promise we thought he had.”
Phil’s Magic wore down stable companion Prince Of Scars to give trainer Sandra Hughes a one-two in the Irish Racing Writers Kingsfurze Novice Hurdle.
Having opened his account over timber at the fourth attempt over the course and distance last month, Phil’s Magic was the 6-4 favourite in the hands of Johnny Burke, with Leopardstown winner Prince Of Scars much bigger odds at 10-1.
Phil’s Magic travelled particularly well throughout and although Prince Of Scars refused to go down without a fight, the market leader won the argument by half a length.
Henry Higgins stayed on to finish third.
Burke said: “I love him. He won well here the last day and he’s going to make a hell of a chaser.
“He really quickened up off the bend and when it got to a battle he battled. He got under the last and had every chance to finish second, but he galloped to the line and through it.”
Fine Rightly toughed it out for Davy Russell in an attritional Naas Directors Plate Novice Chase.
Stuart Crawford’s seven-year-old made it third time lucky in a handicap at Ayr in January and was a 6-1 shot stepped up to Grade Three level.
Russell sent his mount to the lead heading out for the second circuit and had his opponents in trouble from the home turn.
It was hard work from the final fence in the testing conditions, but Fight Rightly had enough in the tank to hold off Rule The World by a length, with Azorian and Lots Of Memories both close up.
Hot favourite Noble Emperor was an early casualty.
Crawford said: “His pedigree is all staying and hopefully one day there could be a real nice chase in him.”
Russell swiftly completed a double aboard the Eddie Harty-trained Fletcher in the Naas Supporters Handicap Hurdle.
The six-year-old was a smart winner at Leopardstown in January, but subsequently finished down the field at Clonmel before falling at Fairyhouse last month.
A 20-1 shot for his latest test, Fletcher was played late by the former champion jockey and finished powerfully to get up and beat consistent mare Mrs Mac Veale by half a length.