#4 Sam Querrey vs Diego Schwartzman
Some months before, Sam Querrey beating anyone outside the top 5 wouldn't be an upset. But his game has declined since by some distance, so it was a great result. Yes, his game was helped massively as he played such a short man in Schwartzman, but mentally that win wasn't straightforward.
The American used his big serve to great effect, getting 3 comfortable holds in Set 1. That helped him as he used the momentum to break in the next game. But, the Argentine broke right back to halt the flow.
Querrey didn't fret though, putting pressure on his opponent's 2nd serve to break again. He held his serve to win the set and continued the same tactic in the next set. He failed to convert 3 break points at the start. But, in the 7th game, he broke serve from 30-0 down.
Sam Querrey served out the remaining games to announce his revival, to the world and to himself.