The U.S. Tennis Association announced the appointments Thursday.
Friemel takes over for Brian Earley, who retired after 26 years as tournament referee in Flushing Meadows.
The job includes making sure rules and procedures are followed, serving as final authority on all officiating matters and approving officials' match assignments.
Friemel has been head of officiating for the International Tennis Federation since 2014 and will retain that post.
He was chief umpire at the U.S. Open from 2016-18.
Garner, the chair umpire for 18 Grand Slam finals, now will oversee selection, training and scheduling of U.S. Open officials.
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