Top-ranked Rafael Nadal advanced to the second round of the U.S. Open when David Ferrer retired in the second set of their all-Spanish matchup.
Nadal won the first set but trailed 3-4 in the second when Ferrer had to stop because of injury.
The defending champion said he felt bad for his Davis Cup teammate, whom he beat for the 2013 French Open title.
Ferrer said afterward it was his final Grand Slam match and he was sorry he wasn't able to finish it.
Andy Murray pumped his fist and then did it again, exulting as if he had just won his match.
Actually, he was still one point away. But considering all the pain Murray has experienced in his hip, sprinting toward the net to chase down a drop volley and put it away was something to celebrate.
Murray and Stan Wawrinka, a pair of former U.S. Open champions, both were winners Monday in their return to the tournament after having to miss it last year.
Wawrinka ousted No. 8 seed Grigor Dimitrov 6-3, 6-2, 7-5, while Murray eliminated James Duckworth 6-7 (5), 6-3, 7-5, 6-3.
Both felt good physically, though not quite as good about their chances of contending for the title.
Murray hadn't played in a Grand Slam tournament since Wimbledon last year, before needing hip surgery that has limited him to just eight matches this year. He said before the weekend it wasn't realistic to think he could win this U.S. Open, and the 2012 champion was asked after the match what would have needed to happen for him to change his tune.
"I would have been able to train and practice a lot more than what I have done. I would have played more matches in the buildup to the tournament. I mean, there's many, many things that I would have wanted to change to be considered a contender," he said.
"I don't think anything changes after today. I think I'm still just taking it one match at a time. Yeah, I mean, this is the first time I have played four sets in 14 months, so, you know, I just have to wait and see how I pull up tomorrow."
Wawrinka has a little more reason for hope. The 2016 champion couldn't defend his title last year and needed two left knee surgeries, but has had some good results this summer. He also eliminated Dimitrov in the first round at Wimbledon, won a couple matches in Toronto before falling to top-ranked Rafael Nadal, and took Roger Federer to three sets in the quarterfinals in Cincinnati.
"I think if you look the last few months, if I separate just my level, just the way I'm playing, the way I'm moving like in practice match or in a match, yeah, for sure my level is really high," Wawrinka said. "I know that, and I'm confident with that."
But the three-time Grand Slam champion acknowledged there were enough questions about his fitness to keep him from thinking too big. He received a wild card into the tournament from the U.S. Tennis Association after his ranking fell too low to qualify directly.
"This year is kind of a transition year," he said. "I want to really play well, win as much as I can, get better ranking, finish the year well."
Murray faces a possible third-round match with 2009 champion Juan Martin del Potro, who's ranked No. 3. He said he'll practice far less now than he used to at Grand Slams, so it's unclear how much better he could get by then. But he improved as Monday went on and hopes that continues.
"I made some quite good moves. Like the second to the last point of the match, I moved pretty quickly up to the drop volley and stuff and kind of maintained my serving speeds throughout the match, as well," he said. "So there was some good stuff, but I think I can get better."
(With AP Inputs)