Theatre Guide opened his account for the campaign after snatching a last-gasp victory in the Unicoin Group Handicap Chase at Cheltenham.
Despite failing to feature in last month's Hennessy Gold Cup, the nine-year-old showed his appetite for a thorough stamina test when getting up on the line to take the Grade Three contest and score for the first time since February.
Approaching the final fence, the Colin Tizzard-trained 15-2 chance looked booked for third as stablemate Bally Longford powered into what appeared a race-winning advantage.
That lead was soon reduced up the hill as both Theatre Guide and long-time leader Perfect Candidate forged on to fight out the finish, with one last lunge seeing Paddy Brennan's mount prevail by a nose.
The winner was trimmed into 20-1 from 25-1 for the Welsh National by William Hill.
Tizzard said: "He just showed he is a thorough stayer. We were disappointed he didn't stay on in the Hennessy, as that's what he does best.
"That was lovely for Jean (Bishop, owner) and my daughter rides him every day. We just felt he didn't go through with the job in the Hennessy.
"Whether it was because it was a higher-class race I don't know, but he definitely went through with it today to the line.
"He is thorough stayer and that (Welsh National) looks the obvious race for him."
As well as the agony of such a narrow defeat, there was an additional sting in the tail for Conor Shoemark, rider of Perfect Candidate, as he was hit with a 15-day suspension by the stewards for using his whip above the permitted level, without giving his mount time to respond and in the incorrect place.
He will sit out December 26 to January 9 inclusive.
Singlefarmpayment received introductory quotes for the RSA Chase in March after gaining his first win over fences in the Jockey Club Ownership Syndicate Novices' Chase.
Having finished in the frame in each of his two previous tries over the larger obstacles, the six-year-old struck at the third time of asking in the extended three-mile-one affair, which was claimed 12 months ago by subsequent RSA winner Blaklion.
Always travelling well in the hands of Adrian Heskin behind the early pacesetting pair of Emerging Force and Relentless Dreamer, the 9-2 shot moved into contention rounding the home turn on the run down to the second-last.
After jumping the final fence with a clear advantage, the Tom George-trained gelding stayed on strongly up the hill to deny Arpege D'Alene by two and a half lengths, earning tags for the Festival contest of 33-1 and 25-1 with Paddy Power and Boylesports respectively.
George said of the former Tom Lacey inmate: "He has done it well. He has taken a bit to get to know as he is quite a highly-strung horse. Adrian Heskin has got to know him well and that is the advantage of having one rider on him all the time.
"We don't do much with fast work with him. He has not worked in six weeks, he just canters every day. He has got into a good routine at home now.
"He was always going to be a better chaser than hurdler. I think the most important thing today was that he has probably improved mentally."
Still Believing (16-1) made her stamina count as she was rewarded for her consistency with victory in the CF Roberts Electrical & Mechanical Services Mares' Handicap Chase.
Dawnieriver looked to have stolen a march on her rivals when forging clear four out but she had nothing left when Adam Wedge ranged alongside after the second-last fence on Still Believing.
Evan Williams' eight-year-old bounded up the hill in resolute fashion to land the spoils by 10 lengths from Treaty Girl.
Williams said: "She's a tough, genuine and consistent mare and she was given a very sensible ride.
"The others probably thought it was the Festival (referring to fast pace), hence the winning margin, but I'll take every Cheltenham winner I get.
"She's a filly that has done nothing wrong all her life and deserved to win a chase or two.
"This is what jump racing is all about. I paid £4,000 for her in a sale here and she's come back and won. We've had a lot of fun with her and hopefully we'll breed a few foals from her that will win races in the future."