ROME (AP) " Rafael Nadal hardly had to get his socks dirty to extend his winning streak to 16 matches.
Back to being the best player on tour over the last month, Nadal advanced from his opening match at the Italian Open on Wednesday when Nicolas Almagro retired in the first set with an apparent injury.
Top-ranked Angelique Kerber, meanwhile, lost her opening match at the Foro Italico 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 last year as well. I had a great year, but I mean, these few weeks I was not playing good. And this year it's the same."
Also, Olympic silver medalist Juan Martin del Potro defeated British player Kyle Edmund 7-5, 6-4; fifth-seeded Milos Raonic defeated Germany 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 countryman. 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 has won consecutive clay-court tournaments in Monte Carlo, Barcelona and Madrid. He's aiming for an eighth title at the Foro Italico as he prepares for the French Open, which starts in less than two weeks.
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.
Also, 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.
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