[4] The highlight of his tennis career was when he reached the semifinals of the 1907 U.S. Championships singles.

Mollenhauer led two sets to one, 5–2 and had two match points when LeRoy hit a shot that looked out but was called good.

Then Mollenhauer suffered from cramps in his arm and legs and lost in five sets.

Mollenhauer was nicknamed "The Flying Dutchman" and played with "dash and vim", according to The Brooklyn Daily Eagle.

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