Marc Skinner's side took the lead after just five minutes at the Chigwell Construction Stadium, as Katie Zelem helped pick the pocket of the Hammers defence, teeing up Williams to smash home past Mackenzie Arnold.
Lucia Garcia and Zelem both went close for the Reds as we looked to build on our advantage in the second period, but the hosts defensive resistance stood firm.
With just five minutes left on the clock, West Ham hit back through Asseyi, with the 30-year-old forward lifting an effort over Mary Earps that looped into the net.
