The 12-year-old beat former Ladrokes World Hurdle runner-up, and 11-4 favourite, Time For Rupert by two lengths in the Class Two contest.
Tranquil Sea, now owned and trained by Warren Greatrex, needed to finish in the first four in a chase over three miles to qualify for the National at Aintree.
And he rolled back the years to evoke memories of his Paddy Power Gold Cup triumph of 2009, much to the delight of Greatrex.
"People thought I was mad buying him but it's great having a horse as good as him around, even if he's not as good as he was," he said.
Earlier in the day JCB Triumph Hurdle fancy was made to work for victory in a Class Four hurdle at Doncaster, the 4-6 favourite getting up to beat Zamoyski by two-and-a-quarter lengths.
Over in Punchestown, Ruby Walsh rode the Willie Mullins-trained Upsie back to winning ways in the punchestown.com Mares Hurdle.
Having stepped up the Grade Three company for her last two outings, and finishing fourth on both occasions, the six-year-old dropped back in class and justified 5-4 favouritism to beat stablemate Urticaire.
And on the day Mullins admitted the target for Cheltenham Festival hot-shot Annie Power will be decided as late as possible, he did show his hand with Champagne Fever.
The seven-year-old, who won the Champions Bumper in 2012 before landing the Supreme Novices' Hurdle last year, has a host of entries for next month's festival.
But Mullins conceded he is leaning towards the Arkle, where Champagne Fever is a 3-1 shot, as he seeks a Cheltenham hat-trick.
© Sportsbeat 2014