Here Comes When has excelled in atrocious conditions to claim a major upset over Ribchester in the Group One Sussex Stakes at Goodwood.

Copious amounts of rain throughout Wednesday (Thursday AEST) morning and afternoon led to second favourite Churchill's withdrawal less than an hour before the race and while 8-13 favourite Ribchester battled bravely in the mud, his brilliance was blunted.

Here Comes When is a proven soft ground performer and was a late entry in the 1600m-race in the hope the forecast rain would arrive in time.

Arrive it did, in bucket loads, and the 20-1 shot knuckled down to see off Ribchester by a neck under champion jockey Jim Crowley.

"He's here for a reason. We were praying that we got the rain and it's come in time. Jim gave him a great ride," trainer Andrew Balding said.

"On this ground, he's stones better than he is on quicker ground."

Ribchester's trainer Richard Fahey said the conditions were against the favourite.

"It's extreme conditions out there, I was very worried," he said.

"It's not for a Flat horse to be racing on, it's National Hunt horses.

"But he's run a mighty race and he showed good heart to nearly get back up.

"We live to fight another day."

The win guarantees Here Comes When a place in the Breeders' Cup Mile at Santa Anita in November.