Nadal extends winning streak to 16 in Rome

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.

Olympic silver medallist Juan Martin del Potro defeated British player Kyle Edmund 7-5 6-4, fifth-seeded Canadian Milos Raonic defeated German veteran Tommy Haas 6-4 6-3.

Russia's Alexander Zverev beat Viktor Troicki 6-3 6-4 and will next face Andy Murray-slayer Fabio 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 (7-1) after more than two-and-a-half hours.

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 per cent now," Nishikori said.