Garin Hess, a Utah entrepreneur, drove to a tennis tournament in Salt Lake City one day last summer expecting to thump his first-round opponent.

Hess had a reason to be confident: He’d never heard of the person he was supposed to be playing. The man listed his hometown outside of Utah and the U.S. Tennis Association rankings didn’t reveal any useful information. It appeared to be his first time playing tennis in nearly a decade. Hess was late for the match, so he asked his iPhone to research his rival.

...