Lewis McGugan had the first effort of the game, but his speculative volley went wide of Bobby Olejnik's left hand post. The goalkeeper was called upon moments later to deny the same player, but it was a routine stop from the former Torquay United shot stopper.
Knight-Percival hooked a volley over the bar following a Camp fumble after a driven Boyd cross. Sinclair was inches away from converting a Knight-Percival knock down on 16 minutes. McGugan dragged a shot wide from the edge of the area on 27 minutes.
Billy Sharp headed a McGugan centre wide of the target on the half hour mark after good Forest pressure. Olejnik made a stunning fingertip save to deny the lively Cox on 39 minutes as Forest came close to the breakthrough.
Bostwick tested Camp with a fierce drive after a good run before the goalkeeper made an even better stop to deny Barnett's glancing header from the resulting corner kick.
The visitors opened the scoring in stupendous fashion as experienced midfielder Andy REID galloped clear and fired a stunning left-foot shot into the top corner.
Barnett tested Camp with a close-range volley after a good cross from Rowe as Posh responded well to the setback of conceding. Sinclair fired over after good work from Little before Forest introduced Jermaine Jenas from the bench.
Olejnik beat away a powerful effort from Billy Sharp on 63 minutes as Forest came close to a second. Rowe was replaced by Lee Tomlin on 68 minutes as Posh reverted to 4-4-2. Danny Swanson saw a 25-yard shot pushed away by Camp by his near post.
Simon Cox was denied superbly by Olejnik on 73 minutes and then he managed to prevent the striker heading home the rebound. Cox fired wide on 78 minutes after beating the offside trap. Boyd fired wide on 86 minutes after a clever Posh free-kick, but time was running out for the boy's in blue.
Final Score: Posh 0-1 Nottingham Forest
Posh: Olejnik, Alcock, Brisley, Bostwick, Barnett, Boyd, Sinclair (sub Berahino 63min), Rowe (sub Tomlin 68min), Ferdinand, Little (sub Swanson , Knight-Percival. Unused subs: Day, Zakuani, Tomlin, McCann, Ntlhe.
Forest: Camp, Harding, Gillett, Collins, Cohen, McGugan (sub Jenas 62min), Reid, Halford, Cox (sub Majewski 82min), Sharp, Ayala (sub Moloney 46min). Unused subs: Evtimov, Moussi, Guedioura,Coppinger.