Seymour finished fifth in the NL in batting average (.294) for the 1907 season. However, an ankle injury prematurely ended his season. His batting average declined to .267 for the 1908 season. That year, he participated in one of baseball's most infamous plays, known as Merkle's Boner, in which the Giants lost the pennant to the Chicago Cubs. In the replayed game between the Giants and Cubs, Giants pitcher Christy Mathewson reportedly waved Seymour to move further back in the outfield; Seymour refused, only to see the ball hit over his head, allowing the Cubs to score three runs on their way to the win. Mathewson later denied waving Seymour back, saying Seymour "knew the Chicago batters as well as did and how to play them."