at time of this comment, most likely the USWNT will be playing Brazil (BRA leads Spain 3 - nil, and I don't expect Spain to equalize with how poor they've been playing).
The concerning thing is that the US has played a lot of extra time and will be fatigued. Conversely, Brazil will be well-rested, having played no extra time in the Olympics. It will be a tough, stressful, and exciting gold medal match!