Djokovic's injury casts grave doubts on his bid to be the first US Open repeat winner since Federer won from 2004-2008, admitting he wondered if he would even get through the first set
Serena Williams nearly crashed out of the rain-hit US Open on Wednesday while top-ranked defending champion Novak Djokovic battled through shoulder pain and Roger Federer advanced despite a weak start.
US eighth seed Serena Williams, seeking her 24th career Grand Slam title to match Margaret Court's all-time record, got a scare before ousting 17-year-old US wildcard Caty McNally 5-7, 6-3, 6-1.
McNally, who hadn't been born when Williams won her first Slam at the 1999 US Open, threatened a struggling Williams from the start before the six-time US Open champion won nine of the last 11 games to escape under the roof at Arthur Ashe Stadium.
"I survived tonight. I'm not too pleased with the way I played at all," Williams said.
"'Serena, you made way too many errors today. What were you thinking?' But I'm alive. I'm happy. I'll do better next time. I promise."
Djokovic, winner of four of the past five Slam titles and 16 in all, was treated for a sore left shoulder throughout his 6-4, 7-6 (7/3), 6-1 victory over Argentina's 56th-ranked Juan Ignacio Londero.
"It was definitely affecting my serve and backhand," Djokovic said of his shoulder injury. "I was definitely tested."
Federer, a 20-time Grand Slam winner who last won the US Open in 2008, rallied to beat 99th-ranked Bosnian Damir Dzumhur 3-6, 6-2, 6-3, 6-4.
The 38-year-old Swiss third seed, who also dropped the first set against Indian qualifier Sumit Nagal in his opener, answered the wake-up call despite 17 first-set unforced errors.
"It's just a bit frustrating more than anything, especially when the level is that low and there are that many errors and the energy is not kind of there," Federer said. "But can only do better, which is a great thing moving forward.
"I clearly have to play better from the get-go."
What the year's final Grand Slam event lacked in quantity, with 22 singles matches postponed by showers, it overcame with quality in amazing matches that challenged three of the greatest stars in tennis history.
One point from surrendering a break into a tie-breaker, McNally won six of the final seven points, the last on a service winner, to capture the first set. Both players made 15 unforced errors.
"I couldn't play too much worse at that point," Williams said. "I was making so many errors off my forehand. I knew I could play better."
Williams broke on a forehand winner in the sixth game of the second set and, after being denied on four set points on McNally's serve, hit a backhand volley winner to force a third set.
Williams opened the third set with a break on the way to a 4-0 edge and cruised home from there.
"She really came out and played really well," Williams said. "She showed no fear."
Djokovic's injury casts grave doubts on his bid to be the first US Open repeat winner since Federer won from 2004-2008, admitting he wondered if he would even get through the first set.
"This is something I've been carrying for quite a while now," Djokovic said. "It wasn't easy to play with the pain, but you have to hope you will get some opportunities and some lucky shots.
"It's not the first time I'm facing this kind of adversity or challenge. It is what it is and I'm just grateful to be on the court."
Five-time US Open winner Federer, who lost to Djokovic in last month's epic Wimbledon final, started poorly but broke early in each of the last three sets and held serve to the end.
"I buckled down and told myself I was going to hang tough and not get broken," Federer said. "That made a big difference."
Japanese seventh seed Kei Nishikori, the 2014 US Open runner-up, beat 108th-ranked American Bradley Klahn 6-2, 4-6, 6-3, 7-5.
French Open champion Ashleigh Barty, the second seed from Australia, eliminated 73rd-ranked American Lauren Davis 6-2, 7-6 (7/2).
Czech third seed Karolina Pliskova, seeking her first Grand Slam title, ousted 202nd-ranked Georgian qualifier Mariam Bolkvadze 6-1, 6-4.
Ukrainian fifth seed Elina Svitolina defeated seven-time Grand Slam winner Venus Williams 6-4, 6-4 and US 10th seed Madison Keys, the 2017 US Open runner-up, trounced China's Zhu Lin 6-4, 6-1.