NEW YORK — So about that inflamed right shoulder that was supposed to hinder Serena Williams at the U.S. Open as she seeks a record 23rd major title: It seems to be just fine. "Definitely feels solid," Williams said. Not sure? There's plenty of evidence. No need to take her word — or her coach's — for it. Look at the way the top seed defeated 47th-ranked Johanna Larsson 6-1, 6-1 Saturday to reach the fourth round and collect the 307th Grand Slam match victory of her career, surpassing Martina Navratilova for most by a woman in the Open era (from 1968) and equaling Roger Federer for most by anyone since 1968.Williams reached 121 mph on a serve. She had a half-dozen aces, bringing her tournament total to 31. She faced only one break point — her first of the tournament — and saved it. She smacked seven return winners. She compiled a 24-5 total edge in winners. "Tennis-wise I think it was very satisfying in all aspects," said her coach, Patrick Mouratoglou. "It's not perfect, of course, But for someone who didn't play much matches in the last two months, I think she's competitive." Now there's an understatement. Serena's sister Venus, seeded sixth, advanced comfortably by beating No. 26 Laura Siegemund 6-1, 6-2. The sisters could meet in the semifinals.Former men's champions Andy Murray and Juan Martin del Potro moved into the fourth round. Murray, who won the 2012 U.S. Open, had trouble in each of the first two sets, but eventually the second seed became more patient during baseline exchanges and took control for a 7-6 (7-4), 5-7, 6-2, 6-3 victory over unseeded Paolo Lorenzi. Murray joins Kyle Edmund — who won Friday — to give Britain two men in the round of 16 at the American major tournament for the first time since 1966.Del Potro's resurgence continued with a 7-6 (7-3), 6-2, 6-3 victory over No. 11 David Ferrer. The 2009 champion missed 21/2 years' worth of majors because of three operations on his left wrist, and he's ranked 142nd, which is why he needed a wild-card invitation to get into the field. Only one American man is left, Jack Sock, after 19-year-old qualifier Jared Donaldson's run ended with a straight-set loss to No. 21 Ivo Karlovic, at 37 the oldest man to reach the fourth round in New York since Jimmy Connors was 39 in 1991.