One of last season's leading novice chasers and among the market leaders for the 2020 Magners Cheltenham Gold Cup, Nicky Henderson's charge was sent off the 1/3 favourite at the Esher track.
However his fencing lacked fluency and he was never able to shake off Now McGinty who helped force the pace throughout.
Santini was a length clear on landing at the last but the runner-up rallied strongly and it took a photograph to separate the pair at the line.
Henderson said: "It was hard work. He wasn't jumping out of the ground like he can do. He jumped 10 fences the other day, and he was electric.
"Out of that ground, he found it hard work. We've got a long way to go. You wouldn't know he is any good at home, so he is probably a hard horse to get proper fit. He was just very rusty. He got away with it today - and we have work to do."
"Nico De Boinville was so clever, because he knew exactly what they were going to do. He said he (Tom O'Brien, on the runner-up) will make it - but he doesn't do anything in front, so I will have to sit on him to try keep the pace honest.
"He is trying to push the leader along to get a better lead I thought he'd got beat, to be honest. We will just have to wind the clock up a bit."
Assessing future targets, the Seven Barrows trainer all but ruled the seven-year-old out of the Ladbrokes King George VI Chase at Kempton Park on Boxing Day.
He said: "It doesn't look like a King George horse. He looked lost in the Kauto Star (over course and distance last year), then ran on nicely to finish on the heels of Topofthegame, and he ran on nicely to finish on top of his heels again (in the RSA).
"That's not his track - so I think you put a line through the King George, and we will just have to see where we go. Everything is considerable, bar a two-mile hurdle race!
"He and (stable star) Altior might well meet - but they won't meet at Kempton, even if Altior goes to Kempton. You wouldn't want to see it as a gallop in Lambourn - because I could have breakfast between the two."
Gold Cup-winning rider De Boinville is confident Santini will take a significant step forward on his next appearance.
He said: "It probably wasn't easy for spectators or punters. He has made harder work of it than I imagined - he was just running in snatches all the time.
"Right now, this time of the season, we are just trying to gauge where are horses are. He is one that will definitely come on for the run, and there is plenty to work on. He has got the ability - we know that - and he will sharpen up a lot for that."
De Boinville senses it may not be until the Cheltenham Gold Cup that Santini is seen at his best.
He added: "I said earlier if we ran the race round Kempton (Kauto Star) again we might have been found out. He wants big, galloping tracks. The Gold Cup course on the New Course at Cheltenham will play into his hands.
"I thought the Old Course played against in a way. He is open to improvement, and there is more to come. It is a stepping stone into the season."
