He played on the ATP Tour for nine years from 1979 , winning one tournament in singles and three tournaments in doubles. In the world rankings, his highest ranking was 39th.
But he is better known as a tennis coach. He trained u. a. John McEnroe , Marcelo Ríos , Evgeni Kafelnikow , and Tim Henman . He made Ríos and Kafelnikow number 1 in the world rankings and Henman made it to number 4 in the world rankings under his direction. Stefanki also coached Fernando González . Under Stefanki, the Chilean made it into the finals in Vienna, the Masters in Madrid and the Davidoff Swiss Indoors in Basel in 2006. González made it to the finals at the Australian Open 2007, his best world ranking was 5th. He was also the coach of his compatriot Andy Roddick . With Stefanki's help, Roddick changed his game so that he could do better on all surfaces. As a result, he made it to the semi-finals at the Australian Open in 2009, the first sixteen at the French Open and the final of the Wimbledon Championships .