The eight-year-old only got to the front in the last half-furlong, but had to withstand the late dash of Irish challenger Cougar Mountain to land the local Group Two spoils by half a length.
Andrew Mullen had rousted Sovereign Debt to ensure a good position near the rail from stall three, before settling him just off the pace.
That proved crucial as Cougar Mountain was compromised from stall 15 of the 16 runners and while Donnacha O'Brien dropped him in in a bid to improve his chances, he only had one rival behind him in the early stages.
The Aidan O'Brien-trained runner stayed on strongly, but the post came too soon and he was half a length behind the strong-galloping winner at the line.
"He's been just done so many times I thought it was going to happen again, so it's really nice for him to get one somewhere abroad," said Nicholls' wife and assistant, Alex Greaves.
"He looked tremendous. He's improved so much since he got here on Sunday, but he did have a bit of a rough flight. They were delayed, but the sun has done him a bit of good."
The winner will now follow a familiar route back home in Britain and try to qualify for the All-Weather Championships at Lingfield on Good Friday, having finished second in the mile event for the last two years.
Sovereign Debt will head there via the Lady Wulfruna Stakes at Wolverhampton, in which he was just touched off last season and won the year before.
"He'll go for the All-Weather Championships, but needs one more run to qualify so he'll probably go back to Wolverhampton first," said Greaves.
Connections of Cougar Mountain were happy with the run and did not want to put the defeat solely down to the draw.
"I don't want to make excuses, but it wasn't ideal," said Coolmore representative Kevin Buckley.
"He ran a good race and Donnacha was happy enough with him."
Roman Legend, trained by Jassim Al Ghazali, was half a length away in third under Harry Bentley, while the Richard Hannon-trained Tupi faded into 15th place after holding every chance in the straight.
