Sock had to retire in his Australian Open qualifying match (Round 1) this year due to an ankle injury. This is his first match since then, so it's quite possible that he's either still struggling with ailments, or feeling out of sorts after taking time off (not sure how much he did in Davis Cup).
He's had numerous injuries in his short career already, so fitness and health seem to be areas due for improvement.
That's not true. He played the AO mixed doubles with Barty which he wouldn't have done if he still had ankle issues. Sock is just overrated and one-dimensional. When things aren't working in a match, he has no plan B.