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Nishioka played a Top 10 only once - Berdych, 0-6 5-7 3-6 on RG clay-court two years ago. Before playing Sock, he lost all four matches against players ranked 11-20: Thiem 2-6 2-6, Kyrgios 3-6 3-6 3-6, Goffin, 5-7 2-6, Bautista 2-6 3-6 3-6 at the Australian Open. Kyrgios is known for his inconsistency, and Sock looked pretty tired in Acapulco, he also retired out of the couples because of his shoulder injury. 20 games against Goffin is sort of a minus, but he's not Nadal's level, and the match against Bautista on the hard court is a good reference because of his similarities to Nadal, and there, Nishioka didn't take over 3 games in either of the three sets.
Rafa lost to a player out of Top 50 only twice, when he retired against Dzumhur and Del Potro, who's worth a lot more. He went over this margin against Marchenko and Kuznetsov in the fast Doha. They're aggressive players who were inspired then, and that was the beginning of the season, the first Rafa tournament played in 2016, then against Bellucci who was at the very edge of Top 50 then (a player who's got ATP titles), and Mannarino (Rafa had 6-1 and a break advantage in the second set). Nishioka doesn't have that aggressiveness nor strength in his hits, he's a player who wants rhythm and rallies, and that suits Nadal better.