One Piece Dream Soccer King

Dream Soccer King is an animated short associated with the One Piece movie Chopper's Kingdom on the Island of Strange Animals. Timeline-wise, the short most likely takes place around the Baroque Works Saga, as Princess Vivi is still with the Straw Hats during this time.

Summary
It is the Grand Line Cup Final, and the special playoff is soon to be decided by a tie-breaker shoot-out, with Coby acting as keeper. Luffy starts the match off by scoring with his Gomu Gomu no Shoot technique. To counter, Buggy uses his Bara Bara technique to break apart and sprint with his reduced weight, only to have the ball curve wide of the post, putting the Luffy Pirates in the lead. Zoro and Hatchan score easily, while Tony Tony Chopper fails. Django, Usopp, and Bon Clay hit the back of the net, tying the score. The Villain All-Stars call a back-up player, Odacchi (who is actually Eichiiro Oda), whose ball is caught with ease by Coby, giving Luffy's team the advantage.

The outcome of the game now depends on Sanji and his incredible prowess in penalties. However, he unbeliveably skies the ball. Just as Buggy victoriously calls a sudden death, an unidentified flying object is reported to be flying over the Grand Line and towards the stadium. The object turns out to be the soccer ball, and the ball lands in the net with a bang, winning the Straw Hats the game and earning them the title of Pirate Soccer Kings.

We set our sights back at the Going Merry, where Luffy is found asleep. As he awakes, he discovers what looks like a brightly-coloured soccer ball. As he charges at the ball with his Gomu Gomu no Shoot technique, it is revealed to the audience that the "ball" is actually Usopp's bottom. The rest is history as Usopp screams in pain (from Wikipedia).