In Boston, Lewis belonged to the outfield trio which included Tris Speaker (CF) and Harry Hooper (RF) and is considered perhaps the best ever in fielding skill. At bat, Lewis was a renowned line-drive hitter who consistently finished in the top ten in most offensive categories despite a short career which was interrupted by World War I (Duffy served as a petty officer in the US Navy).

In 11 seasons, Lewis batted .284 with 38 home runs, 793 RBI, 612 runs, 1518 hits, 289 doubles, 68 triples, and 113 stolen bases in 1459 games.

During his tenure in Boston patrolling left field, Fenway Park featured a ten-foot-high mound that formed an incline in front of the left field wall, now better known as the Green Monster. The young outfielder mastered the incline to such an extent that it was nicknamed "Duffy's Cliff". Sports cartoons of the period often depicted him as a mountain climber making catches amid sheep and snowcaps. The mound was eventually reduced in 1934, long after Lewis had left the Sox, and was not completely eliminated until the field underwent a major renovation following the 2004 season.

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