Someone asked why his foot of the standing leg in a Tree pose (Vrksasana) is hurting so much causing him couldn't stand for long.
I had the same problem. The method I used is similar to what we need in Arm Balancing poses, in the opposite way.
Instead of putting all the weight on the standing leg, try 'claw' the foot firm into the mat - press the ball of the foot and all toes into the mat. And micro-bent the knee of the standing leg, using the strength of the thigh to take some weight off the foot and this will be less stressful for the ankle.
By doing so, the thigh is more tiring but it's a good workout for the leg.