But, when Harper hit first base, his left leg slipped on the bag, causing his knee to shoot straight back.
Harper was unable to put weight on his leg and was carried off the field by his hitting coach and the team trainer.
The 24-year-old is as fearsome a hitter as anyone in the sport, reaching the All-Star Game in four of his first five seasons.
He is among the National League’s leaders in batting average, home runs and RBIs this season.
The Nationals now hold a 14-game lead in the National League East, and it doesn’t seem that there will a be historic collapse for the team.