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.

"The better player won," said Williams, who served up eight double faults, including five in one game in the second set.

She also hit 37 unforced errors.

"It was a great opportunity. It didn't work out the way I wanted it to, but at least I was able to make it to Rio. That was one of my goals."

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

Only three of the top 10 seeds have made it to the last-eight of the women's draw.

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.

However, there were no such problems for defending men's champion Andy Murray and 2008 winner Rafael Nadal who coasted into the third round in straight sets.

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

Wimbledon champion Williams was clearly hampered by the shoulder injury which forced her to skip the Montreal event.

At one point, she screamed in pain, "shoulder, shoulder".

She even served up five double faults in the seventh game of the second set as she suffered her earliest ever loss at an Olympic singles event.

"I didn't expect that I could win. I stayed in the moment, relaxed and went for my shots especially in second set because it was really tight," said Svitolina, the world number 20.

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.

Puig has goosebumps

Muguruza refused to admit that she has found it tough to adjust to being a Grand Slam champion for the first time.

Her results since the French Open in June tell a different story.

Before arriving in Rio, she lost in the first round at Mallorca and Montreal and made just the second round at Wimbledon.

"I didn't find a way to beat her but I am over it. I have doubles and mixed (with Nadal) so I want to make sure I am not left with this bad taste," said the Spaniard.

Puig has had a breakout season, starting the year at 92 and now standing on the cusp of the top 30.

"I still have goosebumps," said Puig.

Second seed Murray raced into the last 16, blitzing Juan Monaco of Argentina 6-3, 6-1.

Nadal, the third seed, was equally ruthless, claiming a seventh win in eight clashes against Italy's Andreas Seppi 6-3, 6-3.

Murray will face Fabio Fognini of Italy for a quarter-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".

Nadal, like Murray looking to become the first player to win two singles golds, faces France's Gilles Simon.

The 30-year-old Nadal also reached the men's doubles semi-finals with Marc Lopez and will for the first time in his career play mixed starting Wednesday.

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.

American seventh seed Madison Keys enjoyed a 6-3, 3-6, 6-3 win over Carla Suarez Navarro and will meet promising Russian Daria Kasatkina.

Two-time Wimbledon winner Kvitova beat Ekaterina Makarova 4-6, 6-4, 6-4.