Spaniard Muguruza lost the first set of her opening round encounter 6-1 to the Czech Republic's Barbora Strýcová before retiring with an illness at the Olympic Green Tennis Center in the Chinese capital.

Stephens, a surprise victor at the US Open at Flushing Meadows in New York City last month, crashed out at the same stage as Muguruza, the 2015 China Open champion.

She was comprehensively beaten 6-3, 6-0 by qualifier and fellow American Christina McHale.

The result for the world number 17 saw her fall to her second successive first round exit since her Grand Slam triumph after she lost to Wang Qiang in the Wuhan Open last week.

"It's very tough, I got a virus in Wuhan," said Muguruza.

"I couldn't perform today." 

Muguruza added: "It's the easy way to think, 'Don't go on the court'. 

"I want to feel like giving the last chance, going on the court, just hit a few balls, see how I'm going to feel, try to turn things around.

"I don't want to retire before the tournament starts."

Elsewhere in the women's tournament, Alizé Cornet of France also knocked out a top player as she beat Germany's 10th-seeded Angelique Kerber 6-4, 6-4.

On the opening day of the men's draw, fourth seed Pablo Carreno Busta of Spain defeated Belgian qualifier Steve Darcis 6-0, 6-4.

Busta's compatriot Roberto Bautista Agut, seeded fifth, is also through to round two after he dispatched China's Ze Zhang 6-1, 6-3.

The tournament in Beijing is due to continue tomorrow.