OP, I feel your pain. We have all been the weakest link (totally sucks) or been in the awkward situation where someone is clearly the weakest link (totally sucks).
One of my teammates said the most insightful thing about this to me recently. I asked her how her doubles match went, and she basically said her partner was overmatched so it was hopeless.
What she said about the experience was pretty cool, though. She said, "Now I know how my partners all these years have felt about me. If they could be nice about it with me, I figured I could be nice about it with her."
So, OP. When the day comes when you are partnered with someone who can't do a thing right, remember your post today.