The Godolphin-owned five-year-old has been restricted to just 10 races, but has never been out of the first three and was winning for a fifth time.
He goes to Dalham Hall Stud on the back of a hard-fought success, despite being off the track after winning the Lockinge Stakes at Newbury in May. The 11-4 shot needed to show stamina and courage in equal fashion but his Brazilian jockey Silvestre de Sousa kept him going to hold off the 2011 winner Cirrus Des Aigles by a neck. Epsom Derby winner Ruler Of The World was half a length away in third.
Cirrus Des Aigles ran an amazing race for a seven-year-old and Ruler Of The World bounced back to his best after disappointing in the Arc two weeks ago.
Of the rest, Hillstar ran too keen early on, while Mukhadram, employing different tactics after being dropped out, stayed on one-paced to finish fifth, with Farhh's stablemate Hunter's Light fourth after making the running.
Farhh's trainer Saeed bin Suroor said: "He's a great fighter - every year he has had problems. He won first time out as a two-year-old and was then injured. We've always had problems with him and he has to have a different programme of training from all the other horses in the stable. You have to look after him really well just to keep him sound. But he looked the best today and he ran his greatest race. He is a great horse and I'm so happy."
Olympic Glory spreadeagled his rivals to win the Queen Elizabeth II Stakes and almost certainly seal the trainers' championship for Richard Hannon.
Wearing blinkers for the first time, the 11-2 chance burst through a gap for Richard Hughes more than a furlong out and powered clear to win by three-and-a- quarter lengths.
The supplemented Top Notch Tonto ran an unbelievable race to finish second, easily justifying Brian Ellison's decision to persuade owner Keith Brown to put up the £70,000 fee. Aidan O'Brien's Kingsbarns also ran a blinder in third with Dawn Approach, in his last race, fourth.
Hannon said: "I spoke to the Queen this morning and she said she was presenting the trophy for the QEII, so I said, 'I'll see you later'. It's funny how it's worked out. It's great."
Those who believed Johnny Murtagh's career in the saddle was winding down when he took out a licence to train this season were proved wrong once again as he steered his own Royal Diamond to a thrilling victory in the Qipco British Champions Long Distance Cup.
The top jockey at Royal Ascot and winner of the King George on Novellist, Murtagh has mastered the Berkshire track this year, but this victory, in the colours of his main backer Andrew Tinkler, meant more than any other.
Sent off at 20-1 after a lifeless run in the Irish St Leger, Royal Diamond appeared to have plenty to find against the likes of the Gold Cup winner Estimate, owned by the Queen, but she never looked happy on the soft ground.
Harris Tweed appeared set for an all-the-way win after seeing off Eye Of The Storm inside the final furlong but Murtagh conjured a magical run from Royal Diamond, who pounced by a nose.
An emotional Murtagh said: "I think this is the best day of my life. I can't thank everybody enough for giving me the horses. It's been an amazing year for me, I really don't know what to say."
The only cloud for Murtagh was the stewards handing him a seven-day ban for misuse of the whip.
Slade Power held off Jim Goldie's Jack Dexter in an exciting finish to the Qipco British Champions Sprint Stakes by a neck
The Uplawmoor trainer said of his horse: "The six furlongs just stretches his stamina, he's like his dad [Orientor] and getting quicker as he gets older.
"He did everything right but had nothing to run with on this side. He ran a great race and we'll come back next year."