BEIJING — Passion, and overcoming injuries, are the key to Rafael Nadal's recent success.

The top-ranked Spaniard won the French Open and the U.S. Open this year, and reclaimed the No. 1 spot from Andy Murray. His next event is the China Open.

"I had some injuries, but good thing is I keep having the passion for the game and the love for what I am doing and the spirit of overcome tough situations in terms of physical issues," the Spaniard said. "Of course, the spirit of improvement: every day when I wake up, going on court with the goal of doing something better than what I was doing."

Nadal will open against Lucas Pouille on Tuesday at the China Open. The Frenchman beat Nadal in their previous meeting last year.

Nadal is 56-9 this season. He leads the tour with five ATP singles titles and will be trying to add this year's China Open title to the one he took home as a teenager in 2005.

Nadal has made the tournament's final in three of his six earlier appearances and has a 20-5 record in Beijing.

In the first round on Monday, top-ranked Garbine Muguruza retired from her opening match with a cold. Muguruza dropped the first set 6-2 and was trailing 2-0 against Barbora Strycova when she decided to stop.

"I couldn't perform today," said the Spaniard, who said she picked up a virus at the Wuhan Open last week but wanted to try to play despite her illness. "I don't want to retire before the tournament starts for me."

Also, U.S. Open champion Sloane Stephens was beaten by Christina McHale 6-3, 6-0.

"It was a tough day," Stephens said. "There's not really much to say about. Obviously not a great match, so just forget about it and try to move on."

In other women's matches, former No. 1 Caroline Wozniacki beat Anastasia Pavlyuchenkova 6-2, 6-2, while Peng Shuai, Elena Vesnina, Elise Mertens, Coco Vandeweghe and Alize Cornet also won.

Men's winners included fifth-seeded Roberto Bautista Agut, Mischa Zverev, Steve Darcis, Fabio Fognini and Aljaz Bedene.