Whitaker set a hot pace over the six jump-off fences to take the lead. Three riders had failed to catch him before Belgium's Jean-Claude Vangeenberghe pushed him back to runner-up on Osta Carrera.
His victory was the first for Belgium in this annual contest and it was gained by just 0.56sec, with a little help from the Dutchman Jos Lansink and Brazil's Rodrigo Pessoa. They advised Vangeenberghe to angle his route through the start to the first fence because it was marginally quicker - and he defeated Whitaker as a result.
Mark Armstrong, 32, who performed well on his first trip here, followed a clear round on the mare Corella with a single error in the second. Despite finishing 13th, he must be a strong contender for the British team to contest the European championships in Spain later this month.
British dressage riders exceeded expectations when finishing third, behind Germany and the Netherlands, in the team contest on Saturday, Laura Fry, on Quarryman, scoring 1,573 points which was 56 more than she had achieved before. Yesterday she was fourth in the Kur To Music.
GERMAN NATIONS CUP MEETING (Aachen, Germany) Grand Prix: 1 Osta Carrera (J C Vangeenberghe, Bel) clear, 35.60; 2 Everest Midnight Madness (M Whitaker, GB) clear, 36.16; 3 Twist du Valon (H Godignon, Fr) clear, 36.43. Last Chance Prize: 1 Almox Grand Plaisir (L Beerbaum, Ger) clear, 44.36; 2 Everest My Mesieur (M Whitaker, GB) clear, 44.64; 3 Planette Folien (T Fuchs, Swit) clear, 44.90. Dressage (Kur To Music): 1 Herrmann's Grand Gilbert (N Uphoff, Ger) 77.2pts; 2 Andiamo (S Rothenberger, Ger) 75.71; 3 Aktion (G Dallos, Hun) 72.96. GB: 4 Quarryman (L Fry) 72.30.