SAN FRANCISCO (Reuters) - America's Cup defender Oracle Team USA on Thursday replaced its tactician John Kostecki in an attempt to reverse its slipping chances of keeping the trophy in San Francisco, after losing four of the first five races to a strong New Zealand challenger.

British Olympic sailing champion Ben Ainslie, the back up skipper on Oracle's practice yacht, will try to rescue the team's floundering campaign on Thursday, as a crew member on the Team's 72 foot catamaran in two planned heats against Emirates Team New Zealand.

(Editing by Alden Bentley)