Ive been struggling in the same piece. This suggestion works well for me.
Any other lessons in guitar tricks could help gettin the strength for the third finger in the 11th fret? When I try to reach the third finger in the 11th fret thats where I lose pressure in the first string on the bar cord in 9th fret.
My thumb just hurts a lot when I successfully play it with all the strength Im using on it to get the bar right in this inversion…
I would suggest practicing and mastering your barre chords. Notice that this particular riff uses barre chord shapes, and if you're having trouble, then check out Fundamentals Level 2, Chapter 5 (barre chords). Work your way through the chapter. As you practice the full barre chords, you will strengthen your fingers, so that when you return to the song, you'll find these shapes easier to play. Hope this helps!!
Keep rockin!
Mike Olekshy
GT Guitar Coach