Serena Williams' hopes of a fifth Olympic gold medal were crushed by Ukraine's Elina Svitolina on Tuesday as the battle for the women's title in Rio was blown wide open.

Top seed and defending champion Williams was clearly struggling with a right shoulder injury as she was stunned 6-4, 6-3 by Svitolina, 13 years her junior.

Defeat for the 34-year-old means that the Rio tournament has lost both its number one players before the quarterfinals after Novak Djokovic went out in the first round.

The Rio Games -- and possibly her Olympic career -- are now over for Serena after she and sister Venus were deposed as doubles champions in the first round.

Williams was joined at the exit on Tuesday by third seed and French Open champion Garbine Muguruza who lost to Puerto Rico's Monica Puig.

Svitolina, 21, will face two-time Wimbledon champion Petra Kvitova for a place in the semifinals.

Muguruza's post-French Open slump continued when she slipped to a worrying 6-1, 6-1 loss to Puig, the world number 34.

Puig, 22, closed in on what would be only Puerto Rico's ninth Olympic medal and will face Germany's Laura Siegemund for a semi-final spot.

Fognini saved two match points to defeat Benoit Paire 4-6, 6-4, 7-6 (7/5). Minutes later, French team officials said that Paire was being expelled from the Games for "flouting rules".

Second seed Angelique Kerber eased past Australia's 2011 US Open winner Samantha Stosur 6-0, 7-5. She next faces British 10th seed Johanna Konta who downed Svetlana Kuznetsova 3-6, 7-5, 7-5.