#2 Nicolas Jarry vs Marin Cilic
One of the few Chilean names in ATP events is Nicolas Jarry, a player who has done very well this year. But, against a beast of a tennis player in Cilic, he had little chances to win. That's exactly how it seemed like for the whole of the first set, when he was decimated 6-2.
Jarry looked like he had erased the memory of the first set when he served well to take the 2nd into a tie-break despite facing a break point twice. In the breaker, he saved a set point on his serve and took the next 2 points to take the set. But, he was broken again during the decider and his competitor served for the match at 5-4.
That's when the Chilean showed his grit to break the game from 30-15 down. After equalling the set he used the momentum to win it on the Cilic serve to win the match. Nicolas Jarry completed the greatest upset of his career.