Yet to be challenged even a tiny bit at this U.S. Open, Serena Williams now gets a sure-to-be-hyped match against one of only three women to beat her all year, Sloane Stephens.

From the moment the women's draw came out at Flushing Meadows, it was clear which potential fourth-rounder was the most intriguing: defending champion Williams against up-and-coming talent Stephens.

"As I always say, I think it will be epic," Stephens said. "I'm really looking forward to it. See what happens."

Hingis loses, still enjoys big stage

At times, Martina Hingis looked like the champion of old — such as when she was pounding forehands or placing crisp volleys right where she wanted them.

At times, Hingis looked like an old champion — most notably, when she double-faulted twice to close out her loss in women's doubles at the U.S. Open on Friday.

Returning to Grand Slam tennis for the first time since 2007, the 32-year-old Hingis went 0-2.

First, she and Daniela Hantuchova lost to the top-seeded and defending champions Sara Errani and Roberta Vinci 6-3, 7-5. Later, Hingis and Mahesh Bhupathi fell 7-6 (5), 7-6 (5) to Chan Yung-jan and Robert Lindstedt in a mixed-doubles match marked by a number of disputed calls in the second-set tiebreaker.

"I always enjoyed the big stage. Even today," said Hingis, who was inducted into the International Tennis Hall of Fame last month. "You see the people that came. They enjoyed it. They really cheered us on. So, I mean, I couldn't ask for more."

Well, except for one thing.

"I mean, I always play tennis to win matches and to win tournaments," she said.

In the first match, while trailing 6-5 in the second set, Hingis served three double-faults, including on the final two points. One of the serves barely reached the net and another was nearly a foot long. Moments after that, she sat down and buried her face in her towel.

"Definitely the nerves. Not playing at a Grand Slam for six years doesn't really help," she said.

And that statement came hours before Williams even had advanced out of the third round by beating 78th-ranked Yaroslava Shvedova of Kazakhstan 6-3, 6-1 in a match that wrapped up at 1:05 a.m. Saturday.

"I'm so excited you guys stayed out for the late-night rendezvous. Thank you, guys, for staying," Williams told the Arthur Ashe Stadium crowd. "I don't think I've ever played this late."

She has dropped a total of eight games through six sets this week. Against Shvedova, she hit serves at up to 119 mph, and produced a 22-3 edge in winners.

Much, much earlier, on a ho-hum afternoon devoid of any truly significant surprises, Stephens reached the round of 16 in New York for the first time by beating 23rd-seeded Jamie Hampton 6-1, 6-3 on Friday.

"Serena is the No. 1 player in the world. She's possibly the greatest player of all time. Sloane is Sloane. You know, she's making her own name. She's top 20 in the world for a reason," Hampton said. "They're both great players, both great competitors."

Hampton's blase summation of a Williams-Stephens matchup: "I don't really make too much of it."

She might be the only one.

"It's something," Stephens said, "I think everyone is looking forward to."

And why not? Williams is 32, seeded No. 1, and owns 16 major titles. Stephens is 20, seeded 15th, and already carrying the label of "Next Big Thing" in American tennis. Not only that, but Stephens surprisingly won their Australian Open quarterfinal in January, one of only four losses in 67 matches for Williams in 2013 (Victoria Azarenka beat her twice, and Sabine Lisicki once). Oh, and then there's this: Stephens found herself in a bit of a brouhaha this year over less-than-flattering comments she made to a reporter about Williams.

"That's all old news now, and we've moved on. We're fine, so I think that's all that matters," Stephens said Friday.

Asked about her relationship with Williams, Stephens replied: "Obviously, we're co-workers. We're Fed Cup teammates. But other than that, everything else is private. It's fine."

Aggressive Serena

They've played twice in the past — both in January, both on hard courts, both in the quarterfinals. Williams won 6-4, 6-3 at the Brisbane International. Three weeks later, Stephens came back for a 3-6, 7-5, 6-4 victory in Melbourne, where Williams was dealing with an injury.

"She's very aggressive. She stays on top you of you. Doesn't give you any room to breathe. She's intense. She knows what she wants to do out there. That's why she's No. 1," Stephens said.

Thinking back to their previous matches, Stephens added: "It was very important for me the first time to just even get out there and be like, 'OK, it's not as scary as I thought it would be.' I think being able to have played her a couple times before, I'm excited to get back out there."

Williams probably is, too, given the way she responds to disappointments such as her Australian Open loss. Since a first-round exit at last year's French Open, Williams has won 94 of 99 matches and earned 13 titles, including at three of the past five Grand Slam tournaments.

"It's going to be tough. Sloane's playing so well," Williams said. "Regardless, there's definitely going to be an American in the quarterfinals."

Her match against Shvedova began at nearly midnight because it followed 2001 champion Lleyton Hewitt's stirring 6-4, 5-7, 3-6, 7-6 (2), 6-1 comeback victory over 2009 winner Juan Martin del Potro, which lasted more than four hours.

Stephens has reached at least the fourth round at all four Grand Slam tournaments this season, including a semifinal run at the Australian Open and a quarterfinal berth at Wimbledon, losing to the eventual champion in both.

"She shows up to play at the Slams, for sure," Hampton said.

While Stephens' record at major tournaments this season is 15-3, she is only 17-15 at other tournaments, with six losses in the first or second round.

"The Grand Slams — it's just showtime, I guess," Stephens said, shrugging her shoulders. "What can you do?"

Li Na exacts revenge

Li Na looked like a Grand Slam champion, while Laura Robson looked like a promising teenager.

The fifth-seeded Li avenged her third-round upset loss to the young Brit at last year's U.S. Open, winning in straight sets Friday at the same stage at Flushing Meadows.

Li, the 2011 French Open champ, rallied from down a break in the second set for a 6-2, 7-5 victory. A year ago, Robson's upset of Li was the biggest victory of her breakthrough run. Meanwhile, it was the third straight frustrating U.S. Open loss for the Chinese star.

Li was nervous after she noticed that her draw was a repeat from last year. A pep talk from coach Carlos Rodriguez eased her anxiety.

"After the talk I was feeling much, much better," she said. "Because before I never try to share the feeling with the team."

Agnieszka Radwanska needed nearly two hours to slip past No. 32-seeded Anastasia Pavlyuchenkova 6-4, 7-6 (1). The third-seeded Pole faced 10 break points, but Pavlyuchenkova was able to convert only two, while Radwanska was 3-for-5.

Robson was ranked 89th coming into last year's tournament. She had never advanced past the second round at a Grand Slam event or defeated a top-10 opponent.

That all changed when she upset major champions Kim Clijsters and Li back-to-back to make the fourth round. Now 19, Robson was seeded 30th at Flushing Meadows and coming off a fourth-round run at Wimbledon.

Few breaks

Li had 34 unforced errors in their match a year ago, and she lamented then that the free points lifted the teen's confidence. This time, Robson never had much of an opening.

"She served very well today and I thought she was returning really deep," Robson said. "You know, there wasn't a lot I could do in some points."

Li surprised herself with 11 aces Friday, including one on a second serve on match point, and won all nine points when she went to the net. She's back in the fourth round at the U.S. Open for the first time since 2009.

Her run over, Robson now must face an unfortunate reality for many teenagers: getting her wisdom teeth out.

"All the other girls in the locker room are telling me their horror stories: 'Oh, yeah, I pulled my gauze out and it was just blood,"' Robson said. "So that's not too nice."