Martina Hingis and Roger Federer are planning to play mixed doubles together at the Rio Olympics 2015

Hingis is a former No.1 in both singles and doubles with 20 majors to her name - five in singles, 11 in doubles and four in mixed; Federer is a former No.1 in singles with 17 Grand Slam singles titles, the most of all time in the men's game. 
Hingis will be going for her first Olympic medal; Federer has two of them to his name, a gold in doubles in 2008 (alongside Stan Wawrinka) and a silver in singles in 2012 (Andy Murray won the gold). And they've played mixed doubles together before. 
They were the Swiss team at the Hopman Cup exhibition event in January 2001, almost 15 years ago, and they went 3-1 together - they won all three of their round robin matches easily in straight sets before falling to Jan-Michael Gambill and Monica Seles in a match tie-break in the final, 2-6, 6-4, 7-6(5). 
They still won the final tie, though, 2-1.