There is arguably no harder task in sports than hitting a major-league fastball. That’s especially true for the fastest of fastballs, many of which now exceed 100 mph. Yet the player who excels at this more than any other isn’t some towering slugger. At first glance, he looks more like a Little Leaguer.

Jose Altuve, the 5-foot-6 star second baseman for the Houston Astros, has become one of the best hitters in baseball by punishing the pitchers that throw the hardest. He is batting .433 on pitches clocked at 94 mph and above...