Former champion Juan Martin del Potro had to set friendship aside on Friday to post a 6-2, 6-1, 7-5 victory over Diego Junqueira in the second round of the US Open. On paper the 18th-ranked del Potro, the 2009 US Open winner, faced an easy task against the 104th-ranked Junqueria, who has toiled on the lesser Challenger circuit for much of this year. But the two grew up together in Tandil, Argentina, and del Potro said that made for a tricky encounter. "It's very tough to play against a friend," he said. 'We live in the same town, we know each other since we were five years old." Del Potro did what was necessary however, to stay on track in his return to Flushing Meadows, where he was unable to defend his title last year because of injury. No one knows better than Roger Federer that del Potro has the game to win a US Open. His defeat to the Argentinian in the 2009 final ended Federer's run of five straight US titles. The key for del Potro, Federer said, would be to get himself into the later rounds without too much wear and tear. Del Potro was following the Federer formula until he hit a speedbump against Junqueria, who nabbed a service break as his compatriot served for the match at 5-4. But del Potro immediately broke back, then had to fight off another break point in the final game before Junqueria lofted a lob long on match point. "Diego is a warrior," del Potro said. "He fights til the last point. Today he made an unbelievable match and it was very tough close in the end." Del Potro's path has opened up with the withdrawal of potential fourth-round opponent Robin Soderling, the sixth seed from Sweden who pulled out of the tournament because of illness. If he stays the course del Potro could find world number four Andy Murray waiting in the quarter-finals. Murray reached the title match here in 2008 to become the first Briton in a Grand Slam final since 1997. Since then the pressure has been on to end Britain's wait for a Grand Slam champion, but he has been unable to do better than runner-up finishes at the last two Australian Opens. He'll try to keep his latest campaign going when he tackles Dutchman Robin Haase for a third-round berth while defending champion Rafael Nadal took on French marathon man Nicolas Mahut. Mahut, best known for his epic encounter against John Isner at Wimbledon last year, rallied from two sets down to get past qualifier Robert Farah in the first round. Nadal was confident that at least he wasn't in for a 11-hour match such as Mahut and Isner served up at Wimbledon. "That's not going to happen," he said. Women's second seed Vera Zvonareva, the 2010 runner-up, led the way into the last 16 with a 6-4, 7-5 victory over Spain's Anabel Medina Garrigues. Third-seeded Russian Maria Sharapova was also vying for a fourth-round berth, taking on Italian Flavia Pennetta. Sharapova, whose three Grand Slam titles include the 2006 US crown, is trying to build on her run to the Wimbledon final, her first appearance in a major final since Australia in 2008. Other women's seeds in action included Australia's ninth-seeded Samantha Stosur and 13th-seeded Peng Shuai of China -- who faced a tough contest against 19th-seeded Julia Goerges of Germany. Germany's Wimbledon semi-finalist Sabine Lisicki tackled young US hope Irina Falconi. Rising US talent takes center stage in the night session on Arthur Ashe Stadium, where 18-year-old Jack Sock was to take on America's former world number one Andy Roddick. Like Roddick, Sock hails from Nebraska and his 2010 US Open junior title was the first for an American since Roddick's in 2000. "Well, I know know he's full of piss and vinegar and he's from Nebraska," Roddick said. "Sounds a little like an 18-year-old I knew once upon a time." Christina McHale, the 19-year-old from nearby New Jersey who is poised for a Cinderella run after toppling eighth-seeded Marion Bartoli, was to take on 25th seed Maria Kirilenko of Russia.