He is one of three Stefanki brothers to have played on the varsity tennis team at the University of California, Berkeley, from 1977 until 1979 under coach Bill Wright.

He is more renowned as a tennis coach, having trained such players as John McEnroe, Marcelo Ríos, Yevgeny Kafelnikov, and Tim Henman amongst others.

Ríos and Kafelnikov both achieved their number 1 tennis rankings while under his guidance, and Henman reached a career-high of 4 under his tutelage.

Under Stefanki the Chilean reached back-to-back finals in Vienna, the Madrid Masters and Basel in 2006 and the 2007 Australian Open final —a tournament in which he eased past the likes of Lleyton Hewitt, James Blake, Rafael Nadal, and Tommy Haas— which saw González, 26, reach a career-high of #5 in the ATP rankings.

He is credited with improving Roddick's tactics and all-court game and helping him reach the semifinals of the 2009 Australian Open, the fourth round of the 2009 French Open for the first time, the 2009 Wimbledon final for the third time, winning the 2010 Sony Ericsson Masters in Miami and the 2010 Brisbane International in Brisbane, Australia.