In a twenty-year career, Mercur was an inconsistent performer, but at his best had a victory over Bill Tilden (at a tournament at Rye in 1928) to his name.

[1] Mercur made his debut at the U. S. Championships in 1921 and lost in round one to that year's finalist Wallace F.

Mercur won the New York State Championships in 1928 defeating Wilmer Allison in the final in three straight sets.

The following season he won the Southampton Invitation on Long Island N.Y. for 1929 defeating John Doeg in the final in three straight sets.

Also in 1929, Mercur won the Florida East Coast Championships defeating Allison in the final in a close five set match.