By Jon Wertheim When the tennis legend retired from competition more than 20 years ago, it was assumed that she would keep a low post-career profile. But even that might be understating her activity in the 21st century. Each summer Sports Illustrated revisits, remembers and rethinks some of the biggest names and most important stories of our sporting past. Come back all week for more Where Are They Now? stories. Tracy Austin did a double take. It was the spring of 2019, those halcyon pre-COVID-19 days, and Austin, once the No.1–ranked player on the WTA Tour, sat in the stands at the USC tennis…