Mohaather finished fast and late to win the Qatar Susses Stakes for Jim Crowley and Marcus Tregoning.
The son of Showcasing was short of room two furlongs out and had to be switched to the centre of the track while others were launching their runs.
However Crowley didn't panic and his partner showed an electric turn of foot to roll past Siskin and catch leader Circus Maximus (6/1).
"He's an absolute aeroplane. I rode him a bit more forward today and doing that found myself in a pocket and got shuffled back. He's done it the hard way, was taken out the race and had to come around them and still won well," the winning rider told ITV Racing.
"Everything went wrong and had I sat and ridden him more confidently he would have been more impressive.
"This is a serious win. You're talking about the best milers in Europe. I'm not saying he made them look ordinary but he won very, very well."
It was a first Group One win for Tregoning since Sir Percy's Derby success in 2006 and he said: "I've had to wait a long time but we haven't quite had the horses. When I left Lambourn we didn't have many and had to start again from the beginning.
"Thankfully Sheikh Hamdan supported me well and I have to give a big plug to Angus Gold who found this horse as a yearling and asked me to go and look at him. I did and thought 'oh no he's quite small' but then I thought Dominica was 15 hands and won a King's Stand for me.
"I thought we'd give it a go as he's a good judge and we liked his pedigree. It was a tactical race, we thought it would be and I don't blame anyone for that, it's just racing. He struggled to get out, he isn't the biggest, but Jim kept calm in the knowledge he has that massive kick.
"If he'd got out earlier he'd have won very easily - he won easily anyway. He's a lovely horse. We love Goodwood and it's a big day for the team, a big day for Sheikh Hamdan who is a very good owner to train for. He takes adversity very well and it's instilled something in me. It makes you a better trainer."
Connections of Circus Maximus were delighted with the performance of the Aidan O'Brien-trained runner-up.
Paul Smith, representing Coolmore, said: “He ran his heart out and you get that every time with him. He is a solid Group One horse and he has run a great race. Ryan was very happy with him and we are delighted.
“It was goose bumps just looking at them around the parade ring as there were some quality individuals there. We will get him back and see how is and see what the lads want to do with him – he is going to be a competitive horse wherever he goes.”
Ger Lyons’ Siskin may have lost his unbeaten record, but Teddy Grimthorpe – racing manager to owner Khalid Abdullah – was far from downbeat.
He said: “He has run a really good race and we have no complaints.
“He has come out of it as the best three-year-old miler, so that is a positive. He has had every chance and has run really well.
“It was a good race. We are going to discuss a whole lot of options for him. We will see how is and see what Prince Khalid wants to do.
“There are a number of options for him which are pretty obvious. We’ve got a bit of time to think about it and see how he comes out of the race. It is only his second race of the year and there is still more to come from him.”
Echoing those sentiments, Lyons said: “I am delighted with the run from Siskin. He is the best three-year-old. He came to win his race and he was outstayed by two older horses. There were no excuses and that is all I asked for coming into the race, that we would have no excuses.
“Colin (Keane) said the ground blunted his speed a bit. One thing we have learned is he is a proper miler and the faster the ground the better. There will be no decisions made, but he probably has Breeders’ Cup written all over him.”
