Riding the 12-year-old chestnut gelding Casper De Muze, Guy produced one of only two clear rounds to finish more than 2sec faster than Harriet Nuttall in second. Guy's horse had lost a shoe half way round the course but continued at a strong pace to cross the finish line in 96.36sec.
"I knew I was going to win," said Guy afterwards. "The horse is a speed specialist, he has won everywhere and he came here for the Speed Derby. Hickstead is my favourite show of the summer and I like winning in the main ring."
Harriet Nuttall is having a good week at the Equestrian.com Hickstead Derby Meeting, having also finished second in the Bunn Leisure Derby Trial on A Touch Imperious on Friday. In today's class she was runner-up once again, this time on the grey gelding Silver Lift.
"The only thing he lacks is that he's small so he doesn't quite have the stride coverage of some of the other horses but he does have turbo and he is really quick off my leg," explained Harriet.
Both Guy and Harriet are among the favourites for tomorrow's Equestrian.com Derby, Hickstead's most famous class. Harriet rides A Touch Imperious, while Guy has two chances - Depardieu van t Kiezelhof and Skip Two Ramiro.
'Skippy' has an amazing record in the class, having been placed in the Derby most years since 2003 and winning the title in 2010. "If I ride Skippy well he could definitely win it," says Guy. "He's 18 now and only jumps four times a year but as long as he's enjoying it he can keep doing it. Last year he didn't feel quite right, but we've got him a lot fitter this year."
Earlier today, Ireland's Daniel Coyle and Zuidam won the Tom Hudson Derby Trophy. Twelve horses had gone through to the jump-off, and British hopes were raised when Jay Halim netted a fast clear on Goodman For Fun II. But 19-year-old Daniel came in next and proceeded to knock a second off Jay's time to take home the prize.
"It's my first big win at Hickstead, and I've only had the horse a couple of months so I'm delighted," said Daniel.
Jay has collected a number of placings this week, and the eventer-turned-showjumper's ride Goodman For Fun II has been one of the most consistent performers at the show.
"It's been a fairy tale for me to be jumping here at Hickstead," said Jay. "Though if someone had told me at the beginning of the week that I'd win a second and third place in the International Arena I'd have been delighted - but now I'm a little bit frustrated to keep coming so close to winning!"