Both teams had chances to open the scoring in the first half as Tullis-Joyce saved from each of Alyssa Thompson, Erin Cuthbert and Lauren James - the latter two via outstanding stops - while Jess Park forced Hannah Hampton into a scrambling save at the other end before blazing a decent chance over the bar.
Chelsea largely dominated that first half, but United grew into the game after the break, and after Melvine Malard had a goal-bound header cleared off the line by Veerle Buurman, Park hit the post with a rasping, 25-yard effort.
However, it was Sonia Bompastor's side who eventually took the lead as substitute Kerr hooked a bouncing ball into the bottom corner. United were only behind for three minutes, however, as their own replacement, Awujo, poked the ball home after some pinball inside the Chelsea box to ensure extra-time.
The home side reasserted some control and took the lead when Girma was the quickest to react after her United States team-mate Tullis-Joyce tipped Buurman's header onto the post. United did push for a second equaliser, but couldn't produce any clear chances to keep their hopes of another trip to Wembley alive.
GOAL rates United's players from Kingsmeadow...



