Could Rio go down in Olympic history as the No-Show Games?

To the long list of controversies and problems plaguing this summer's Olympic Games in Rio de Janeiro, here's one more that's a case of addition by subtraction, and gathering steam as the Aug. 5 opening nears: No-shows from top athletes.

Dominic Thiem, at 22 the youngest top-20 ranked player in men's tennis, announced Thursday he would pass on a trip to Rio this summer. "There are many reasons. Maybe I'm going to explain it another time," the Austrian told reporters after dispatching Roger Federer (who, coincidentally, met his wife Mirka when they were both playing in the 2000 Olympic tennis tournament in Sydney, Australia) in straight sets at the Italian Open.

Another tennis possible in Spain's Feliciano Lopez has announced he won't go. Five golfers including Australia's Adam Scott, ranked seventh in the world, and compatriot Marc Leishman, who cited fears of the Zika virus, have declared themselves out. Same goes for a pair of top Kenyan marathoners in world record-holder Dennis Kimetto and 2012 bronze medalist Wilson Kipsang.

They may well be merely just the start. The BBC reported Thursday that the World Anti-Doping Agency will declare Kenya non-compliant, just days after a new law raised hopes of an Olympic pass for the country's athletics federation. Kenya led all countries with seven gold medals at last year's world championship -- at the same time, though, 40 of the country's athletes have tested positive for banned substances over the past five years. Meantime, Russia's track and field participation at the Games is in doubt due to a doping scandal. Between them the two countries won 13 medals at London 2012, including 11 by Kenya in distance running.

Less trumpeted but potentially just as impactful is the rift In European basketball. A dispute between FIBA Europe and its national federations over Euroleague participation has been simmering for years but now sanctions have been threatened against countries like 2012 silver medalist Spain and Lithuania, among others, that would keep them out of Rio. For now, individual players like Chris Paul and Anthony Davis of the U.S. have declared themselves out of the running.