NEW YORK -- A bad back prevented Roger Federer from getting ready for the US Open the way he prefers to prepare for a Grand Slam tournament. And it has showed so far.
Federer blamed a lack of proper practice after making an uncharacteristic 68 unforced errors and being forced to go five sets again before coming back to edge a cramping Mikhail Youzhny 1-6, 7-6 (3), 6-4, 4-6, 6-2 on Thursday in Arthur Ashe Stadium.
It is the first time in his long career that the 36-year-old Federer has played five-setters in both the first and second rounds at a major tournament.
"I knew I was going to maybe struggle early on. Maybe I struggled more than I would have liked to. But I'm still in the draw, which gives me a chance. I still believe I'm going to pick up my game and become just more consistent because I'm not playing all that bad," Federer said. "It's just that I'm going a bit up and down in waves throughout the match."
Given that Federer entered the day with a 16-0 career record against Youzhny and a 16-0 mark in the US Open's second round, one might have thought that their match would be a mismatch.
Think again.
"He's also a real man who plays tennis," Youzhny noted. "He's not a god."
Well, OK, that's true. But remember: Federer did not drop a single set en route to his record eighth Wimbledon championship in July. And that he is 37-3 this year with five titles, including two at majors to raise his record total to 19.
Second on that list, with 15 majors, is No. 1-seeded Rafael Nadal, who trailed by a set and a break before figuring things out and beating 121st-ranked Taro Daniel of Japan 4-6, 6-3, 6-2, 6-2 in Ashe.
Daniel, who has never been past the second round at a major tournament, was better in the opening set Thursday night and went ahead 2-1 in the second. But Nadal finally converted a break point on his sixth try of the match in the next game to get to 2-all and began to calibrate his big forehand better.
The No. 3-seeded Federer won five consecutive US Open championships from 2004 to 2008 and also was the runner-up twice, including two years ago. But he missed last year's tournament while taking off the second half of the season to let his back and surgically repaired left knee fully heal.
That back, an off-and-on issue for years, flared up again while Federer was losing in the final of the Montreal Masters in August. He didn't get to fully work on returns or serves, in particular, as the US Open approached. That lack of training and the resulting lack of timing -- rather than pain from his back -- is what Federer said caused him problems Tuesday night in the first round against 19-year-old American Frances Tiafoe and again against Youzhny, a former top-10 player who reached the 2006 and 2010 US Open semifinals but is now ranked 101st.
"I'm pretty confident that I'm only going to get better from here. That's a good thing," Federer said. "Because I've played a lot, I definitely found some rhythm now."
He did falter repeatedly Thursday, though.
Federer let a lead slip away in the second set and got broken while serving for it at 5-4.
"He helped me to come back," Youzhny said.
Federer stumbled again while serving for the fourth set at 5-3, but he responded to a break by breaking right back.
Still, Federer's unforced errors continued to mount in the fifth set -- 11 in the first four games alone, including a badly shanked forehand on his first break point at 2-1, a netted backhand on his second and a long forehand to let Youzhny hold there. Eventually, Federer nosed ahead, aided by the considerable dip in the 35-year-old Youzhny's level of play.
While Federer played his first-round match Tuesday under the roof in Ashe, Youzhny's opener was postponed until Wednesday because of rain, and he blamed that for his fatigue. His legs started cramping late in the third set Thursday and then reached other parts of his body, even his fingers, by the end. That made it hard to move forward or to jump normally while serving.
At 1-all in the deciding set, Youzhny collapsed to the court, grabbing at his right leg after whiffing on an attempted swat at Federer's lob. Youzhny stayed down for a few moments, then grimaced and limped around for the rest of that game.
Afterward, Youzhny chuckled at the mention of having lost all 17 matches against Federer, even leaning back in his chair and joking about how he would have completed the upset if Grand Slam rules were different: "I beat him in three sets now -- but we played five sets. But come on, if we played three sets, I already beat him!"
Federer, naturally, preferred to look at matters from a different perspective.
"I find my way," he said. "I don't panic."
Earlier Thursday, Grigor Dimitrov became the latest top-10 men's player to bow out of the tournament, upset in the second round by 19-year-old Russian Andrey Rublev.
The 53rd-ranked Rublev eliminated No. 7 Dimitrov 7-5, 7-6 (3), 6-3 to reach the third round at a major tournament for the first time.
Dimitrov is a three-time Grand Slam semifinalist who had been playing well leading into Flushing Meadows. He had won 13 consecutive sets heading into Thursday, dating to the Cincinnati Masters, where he won his first title at the tour tier just below the majors.
Against Rublev, Dimitrov double-faulted 11 times and converted only 2 of 10 break points.
Dimitrov's exit raises to six the number of members of the ATP's top 10 who are out of the field in New York. No. 2 Andy Murray, No. 4 Stan Wawrinka, No. 5 Novak Djokovic and No. 10 Kei Nishikori all sat out the US Open with injuries, and No. 6 Alexander Zverev lost to 18-year-old Canadian Denis Shapovalov in the second round Wednesday night.
In another early match Thursday, Alexandr Dolgopolov, who was involved in a match at another tournament that's under scrutiny because of unusual betting patterns, eliminated No. 15 seed Tomas Berdych to reach the Open's third round.
Dolgopolov beat 2010 Wimbledon runner-up Berdych 3-6, 6-1, 7-6 (5), 6-2.
Tennis Integrity Unit spokesman Mark Harrison said the group "was made aware of concerns over betting patterns" during the match between Dolgopolov and Thiago Monteiro at Winston-Salem, North Carolina, on Aug. 20. The match is being assessed but is not yet under formal investigation, Harrison said.
Harrison notes that many reasons other than corruption can explain unusual betting patterns.
The Associated Press contributed to this report.