ROME - Rafael Nadal hardly had to get his socks dirty to extend his winning streak to 16 matches at the Italian Open on Wednesday.

Back to being the best player on the tour over the past month, Nadal advanced from his opening match when Nicolas Almagro retired in the first set with an apparent injury.

In the women's draw, top-ranked Angelique Kerber lost her opening match to Estonian qualifier Anett Kontaveit 6-4, 6-0.

Kerber joined Andy Murray, the top-ranked men's player, on the sidelines after Murray's loss to Fabio Fognini on Tuesday.

"Everybody knows I'm not a clay-court specialist," Kerber said.

"I was not playing good on clay last year as well. I had a great year, but these past few weeks I am not playing good."

Also, Olympic silver medalist Juan Martin del Potro defeated Kyle Edmund 7-5, 6-4; Canada's fifth-seeded Milos Raonic overcame German veteran Tommy Haas 6-4, 6-3; and 16th-seeded Alexander Zverev beat Viktor Troicki 6-3, 6-4 and will next face Fognini.

"The stadium will be very loud for him. I'm very sure about that," Zverev said of Fognini. "It's going to be a fun day, a fun atmosphere and great match."

Venus Williams, the 1999 champion, eliminated Lesia Tsurenko 6-4, 6-3.

Nadal was leading 3-0, 15-30 when Almagro bent over toward the red clay court and grasped his left knee.

Nadal went over to Almagro's side of the net to ask what was wrong and consoled his fellow Spaniard.

Almagro then got some medical attention at his chair but quickly retired from the second-round match.

"It's difficult to see what's going on now just after what happened," Nadal said. "He felt something on the knee."

The fourth-ranked Nadal remained on court for a practice session.

Nadal, winner of consecutive clay-court tournaments in Monte Carlo, Barcelona and Madrid, is aiming for an eighth title at Foro Italico as he prepares for the French Open, which starts on May 28.

Nadal will next face 13th-seeded Jack Sock, who beat Jiri Vesely 6-4, 3-6, 7-6 (1) after more than 2 1/2 hours.

The 73rd-ranked Almagro had come through qualifying.

Meanwhile, seventh-seeded Kei Nishikori beat David Ferrer 7-5, 6-2 and will next face Del Potro.

Nishikori showed no lingering pain from the right wrist injury that prompted him to withdraw from the quarterfinals in Madrid last week.

"My wrist is OK now. I assume to be 100 percent now," Nishikori said.

Kerber returned to No. 1 this week despite getting eliminated early in her past two tournaments. She hasn't won a title this year but was able to move up in the rankings with Serena Williams pregnant and taking the rest of the year off.

Also, second-seeded Karolina Pliskova defeated Lauren Davis of the United States 6-1, 6-1, and sixth-seeded Simona Halep beat Laura Siegemund of Germany 6-4, 6-4.

Eighth-seeded Elina Svitolina eliminated Alize Cornet of France 6-4, 7-6 (11), and 15th-seeded Kiki Bertens beat American qualifier CiCi Bellis 6-4, 6-0.

Associated Press