After a shaky start, the 29-year-old broke his No. 122-ranked opponent twice in each of the second and third sets to win the match.
Since defeating Sinner in the Rotterdam final in February, Medvedev is now 6-0 against Sinner in their head to heads.
Khachanov had to dig deep to register a fantastic 7-6 (7-4), 6-4 win over the Greek, who failed to get into his stride against the Russian.
Fritz recorded eight aces and returned three out of four break points in the tie. His win percentages on the first and second serves were 56% and 62% respectively.
The Italian tennis player hit eight aces and returned three out of five break points, and his win percentages on the first and second serves were 85% and 68% respectively.
The Argentine's baseline play caught the world no. 6 off guard, who never really recovered at any point during the hour and 44 minute match.
For van de Zandschulp, it is a momentous occasion as he recorded his first ever top-five win, and will be now looking to go the distance in Miami with his new found confidence.
Alcaraz registered three aces and returned four out of five break points to register a 6-0, 7-6 (7-5) victory on Sunday.
The Brit could only record three aces, and committed one double fault. He could not return a single break point.
Rublev recorded an outstanding 12 aces in the match and committed not a single double fault.