The 2015 South African champion jockey, who rode a double at last year's Shergar Cup meeting at Ascot, gained his first winner during a three-month stint in Britain when steering the Amanda Perrett-trained 7-2 chance to victory by two lengths on just his third ride at the track.
He said: "It's an amazing feeling to have a winner here. It is definitely a tick off the bucket list."
He went on: "I am here for three months and I hope to find a good horse to get on in that time. I was leading the South African championship and had to let it all go, but these opportunities don't come around too often and you have to take them with both hands.
"I am enjoying it and I hope it works out. I'd love to ride at Royal Ascot and Glorious Goodwood and I would love to ride at Epsom. There are so many magnificent tracks and races. It's a horseman's haven here."
Saigon City bolstered his chances of an outing at Royal Ascot when getting the better of a battle of the veterans in the Edmondson Hall Solicitors & Sports Lawyers Handicap.
The Declan Carrol-trained seven-year-old could be in line to join stablemate Santry at the blue riband meeting after he defeated Oriental Fox, who is two years his senior, by two and a quarter lengths to complete a double for Pat Cosgrave.
Ray Flegg, part-owner, said of the 9-4 winner: "We have been waiting for this weather. We wanted to run him in the Chester Cup as that would have been ideal, but it was too firm.
"Pat said he wants further. We have got to talk to Declan first, but the two and a half miles in the Ascot Stakes looks like it could be ideal. The further he goes the better.
"Me and John (Bousfield) also own half of Santry, so it doesn't get much better than this."
Eustace said: "It has taken me 28 years to have my first winner on the Rowley Mile. I don't have many runners at Newmarket as I don't have many that are good enough.
"I didn't have a clue how he was going to run on it with all the rain, but he clearly loved it. Jeff Smith has been a great supporter and I am delighted to have a winner for him."
Having partnered Mr Lupton to glory on his previous visit to the course, French ace Gerald Mosse was back among the winners when steering the Heather Main-trained Rake's Progress to glory in division two of the mile handicap.
Mosse said: "That was nice and easy. He lost his balance and I had to put him back underneath me. He is still a big baby. The ground helped him a lot today."
The John Mackie-trained Art Echo (2-1) got the ball rolling for Cosgrave in the Chemtest Handicap, while G K Chesterton (5-2) from the yard of Charlie Appleby struck in the Discover Newmarket Handicap.