New Zealand's Mark Todd took an unfortunate tumble from Gandalf on the cross-country, ending their chances.
"It was a tough course and you had to go for it," Ostholt said. "I made a stupid mistake with Air Jordan in Luhmuhlen (when they stopped) and so I was a little nervous this time, because he is such a super horse that I wanted to show him at his best."
In addition, their compatriots Andreas Dibowski, Marina Kohncke and Kai Ruder are in 5th, 6th and 8th places.
But defending champion Clayton Fredericks (Australia) is still in third place on Ben Along Time, 4 penalties behind Ostholt in third, and with the horse's renowned prowess in the jumping arena he cannot be ruled out from a historic third title, so the scene is set for a nail-biting finale today.
Dressage leader Oliver Townend departed the fray in the most dramatic and frustrating fashion. Having made the long journey across Europe from Britain to Poland in the hope of adding the HSBC FEI World Cup Eventing title to his lead in the HSBC FEI Classics, his horse Flint Curtis suddenly tripped over on the bend preceding fence 8 and fell, resulting in elimination.
Instead, it was left to compatriot Ruth Edge to fly the British flag, which she did in great style with a determined round on PC Wilson, one of the least experienced horses in the field and with whom she only made the decision to travel to Poland at the start of the week. She is now in fourth place.
And the home crowd had something to cheer about with a great round by Pawel Spisak (POL) on Weriusz, the last combination on course and now lying in 7th place. "I didn't watch very much," he said. "I just went to my car and tried to concentrate on my own job. I did not set out to ride conservatively, but Weriusz is not a large horse and the jumps were quite big for him, so I wanted to look after him."
Denmark's Peter Flarup and Silver Ray are one of the best and fastest cross-country combinations around and, first on course, they made it look easy. But when they came home with 19.2 time penalties, it was obvious that this would be a challenge.
And, as course designer Rudiger Schwarz (Germany) has proved many times at Aachen, he is the master of producing a varied course within a limited space, and one which proves a more influential test than at first sight. "It was a good day. For the Final, the course had to be influential," Schwarz said, "but I also needed less experienced riders to get to the finish."
Ground Jury President Marilyn Payne said that yesterday, she had judged the most exciting dressage of my life. "Today was the most exciting cross-country day of my life. No-one was injured and we were on the edge of our seats. It was a fabulous day for the sport. I can't wait for the Jumping!"
Clayton Fredericks commented that perhaps some riders may have under-estimated the test. "It was an exciting day," he said. "I had a long wait and to see so many good riders make mistakes just added to the tension."
The first major casualty was Germany's Dirk Schrade, lying sixth after dressage on King Artus when they had a refusal at the narrow brush into the sunken road at fence 18, and everyone was disappointed for New Zealand's Mark Todd, seventh after dressage, when he had a fall with Gandalf at the first of the angled Hong Kong Hedges at 20. The grey, who had been going beautifully, shuffled a stride and jinked to the left, decanting his rider.
It wasn't a great day for the Americans, either. Buck Davidson (USA), 8th after dressage, had a run-out with My Boy Bobby at 15c, a narrow chest taken on a tight turn following a bounce of substantial chests. His compatriot, Kelly Prather, had two refusals.
Stuart Fitzgerald, the first South African to compete in the World Cup Eventing Final, had an unfortunate experience when his horse, Classic Diamond Charm, refused point blank to enter the HSBC Water over the house fence at 14; Paul Tapner (Australia) and Tiger Flynne fell at the penultimate fence, a brush corner at 26; and Erwan Le Roux (France) was well and truly submerged in the water when Fidji de Magne Mili landed steeply over the fence in to the second water at 21.
29 horses started the Cross-Country
2 withdrew (Lucinda Fredericks, AUS, with Headley Britannia, 13th after Dressage, and Camilla Spiers, IRL, on Portersize Just A Jiff, 28th after Dressage)
14 clear rounds
5 German riders in the top 8
No one inside the optimum time of 7min 15sec
Michael Jung was fastest, with 0.8 penalties
5 were eliminated
Between 15,000 and 20,000 spectators attended