Victoria Duval pulls a huge upset at the US Open: What happened?

As fan favorite Roger Federer took the first step toward a possible quarterfinal meeting with nemesis Rafael Nadal, an unknown teen from the United States made a Grand Slam breakthrough Tuesday. Victoria Duval, a 17-year-old qualifier who is ranked 296th, pulled off quite an upset, eliminating 11th-seeded 2011 U.S. Open champion Sam Stosur 5-7, 6-4, 6-4.

"A big moment, big stage. Not easy closing any match out, let alone a past U.S. Open champion," Ms. Duval said. "So happy I did it, though."

Duval jumped up and down with arms aloft after pounding a forehand winner to convert her fourth match point. And why not? Duval had never beaten a player ranked higher than 69th, never even faced one in the top 20, and never won a Grand Slam match.

"I know she didn't play her best today, and this is the best I've played in my career, so I'm really excited,"Duval told the Louis Armstrong Stadium crowd. "I just tried to stay in the moment."

Stosur was unhappy with the way she played, including 10 double-faults and a total of 56 unforced errors, 21 more than Duval.

"I'm not going to be a sore loser and say she didn't do anything," the 11th-seeded Stosur said. "But, you know, I think I certainly helped her out there today, that's for sure."

Other seeded women on the way out were No. 17 Dominika Cibulkova, No. 20 Nadia Petrova and No. 31 Klara Zakopalova.

No. 2 Victoria Azarenka, the 2012 U.S. Open runner-up and a two-time Australian Open winner, needed 10 minutes and six break points to take the first game against 99th-ranked Dinah Pfizenmaier, then dominated the rest of the way en route to a 6-0, 6-0 victory.

Her match in Arthur Ashe Stadium came after top-seeded Novak Djokovic began his bid for a second U.S. Open title, and seventh major trophy overall, by beating 112th-ranked Ricardas Berankis of Lithuania 6-1, 6-2, 6-2.

"I played every point like it's a match point," said Djokovic, who recently added former top-10 player Wojtek Fibak as a coaching consultant.

Other straight-set winners included No. 5 Tomas Berdych and No. 10 Milos Raonic. On a day that American men went 5-2, led by No. 13 John Isner and No. 26 Sam Querrey, a handful of seeded players made quick departures. No. 14 Jerzy Janowicz of Poland, a semifinalist at Wimbledon last month, was the most surprising to go, although he was treated by a trainer for a painful back during a 6-4, 6-4, 6-2 loss to 247th-ranked qualifier Maximo Gonzalez of Argentina.

"It was like someone puts a knife through your lower back," Janowicz said.

Janowicz is a volatile character, and that was on full display Tuesday. He pounded two balls in anger into the stands. He swatted one serve underhand. He chucked his racket. He argued with the chair umpire.

Other losers included No. 15 Nicolas Almagro, No. 25 Grigor Dimitrov and No. 28 Juan Monaco.

What about Roger Federer?

At this point in his career, Roger Federer recognizes the importance of a little extra work.

That's why the owner of a record 17 Grand Slam titles, and the man who spent more weeks ranked No. 1 than any other, was out there on a U.S. Open practice court late Tuesday afternoon, putting in some training time shortly after finishing off a 6-3, 6-2, 7-5 victory over 62nd-ranked Grega Zemlja of Slovenia in the first round.

At 32 and holding his lowest ranking, No. 7, in more than a decade, and coming off a stunningly early exit at the previous major tournament — one of a series of newsworthy losses lately — Federer is OK with making some concessions. He insists his passion for tennis is still there.

"I'm in a good spot right now," Federer said. "I want to enjoy it as long as it lasts."

He made it sound, though, as if it isn't as easy to enjoy things the way his results have been going.

Federer entered Tuesday 32-11, a .744 winning percentage that doesn't sound too bad, until you consider his career mark at the start of this season was .816, and he's had years where he went 81-4 (.953). and 92-5 (.948). He's only won one tournament in 2013, which would be great for some guys, but Federer topped 10 titles three times, and hasn't won fewer than three in any season since 2001.

"Clearly, when you win everything, it's fun. That doesn't necessarily mean you love the game more. You just like winning, being on the front page, lifting trophies, doing comfortable press conferences. It's nice. But that doesn't mean you really, actually love it, love it," said Federer, whose streak of 36 consecutive Grand Slam quarterfinals ended with a second-round defeat at Wimbledon against an opponent ranked 116th. "That maybe shines through maybe more in times when you don't play that well. For me, I knew it — winning or losing, practice court or match court — that I love it."

Federer says he doesn't fret about being seeded seventh at Flushing Meadows, a year after being seeded No. 1. Not since 2002, when he was 13th, had Federer been so low at the U.S. Open.

That didn't really affect Tuesday's opponent in Arthur Ashe Stadium. All that mattered to Zemlja, who owns fewer Grand Slam match wins, eight, than Federer owns Grand Slam titles, was that he was facing what he considered an impossible task.

"If he's the seventh seed or fourth seed or first seed, for me, that's totally irrelevant," Zemlja said. "He achieved so much. He's the best player of all time. So I don't think people can actually say something (negative) about the way he's playing. You're losing matches, you're winning matches — that's just tennis, and I'm sure he's going to perform better than maybe he has done in the last few tournaments."

Difficult as things have been for Federer, he certainly remains capable of summoning his best strokes. A bad lower back has bothered him this season, and he's experimented with a larger racket head, but with his old equipment in hand Tuesday, a healthy-looking Federer collected 35 winners and only 16 unforced errors.

Wearing neon-pink-and-gray shoes with a "5'' etched inside a silhouette of the U.S. Open trophy on the right heel — the number of titles he's won in New York from 2004-08 — Federer won 20 of the 21 points he played at the net and 62 of the 80 points he served. To cap the first set, Federer spun a 95 mph ace into a corner. To cap the second, he hit a 118 mph service winner that forced Zemlja into a backhand return so wild that it sailed directly into a guest box in the stands, where Federer's agent happened to catch the ball on the fly. And to cap the third, Federer pressed forward for a swinging forehand volley winner.

"I decided ... to play aggressive," Federer summed up. "I was happy the way I played, you know, overall. I mean, it's a first round, after all."