Poole will take a four-point lead into the second leg of their Elite League play-off semi-final after winning 47-44 at Lakeside.
Brilliant Australian Darcy Ward led the way for the Pirates with 13 points in a clash that went one way then the other.
Back-to-back maximums midway through the night put the Hammers in front but with Scott Nicholls failing to fire as guest for the crocked Davey Watt, it was Poole who finished the stronger.
Kauko Nieminen teamed up with Peter Karlsson for a Hammers 5-1 in Heat five with Grand Prix leader Chris Holder back in third place.
But when Nieminen crashed out at the first bend two races later Ward partnered Piotr Pawlicki to hand the visitors their first maximum of the night and a 23-19 lead.
Lakeside hit back immediately though, Karlsson (11+2) and Nieminen again working well together after captain Robert Mear (9) had teamed up with Kim Nilsson a race earlier.
However, the hosts would not gain another heat advantage on the night as the depth of the Pirates shone through.
Ward took Heat 10 from the 'graveyard' outside lane before Holder (8+1) courageously denied Karlsson another race win.
The outstanding Ward came from behind to lead home a 4-2 in the 12th before Krzysztof Kasprzak (12) teamed up with Holder for a maximum in the pivotal Heat 13 to give the Pirates a four-point advantage.
Mear showed great spirit to take the penultimate race after Karlsson's bike packed up early on, but the veteran showed he was no back number by winning the finale ahead of Kasprzak and Ward to at least keep the Hammers in touching distance heading into the second leg.
The second play-off semi-final between Birmingham and Swindon was postponed due to the weather and the first leg will now take place next Monday.