New York - Novak Djokovic and Andy Murray have met in three of the last four Grand Slam finals, though if they play in the U.S. Open this year, it will be in the semis.

Each took a step forward Friday, shaky at moments and sensational in others, in second-round victories. The top-seeded Djokovic faced two early set points, while defending champion Murray had to go four sets.

Leonardo Mayer, ranked 81st, ran Murray all over the court, but the third-seeded Brit excels at chasing down shots. Murray won the last five games for a 7-5, 6-1, 3-6, 6-1 victory.

"He's a big hitter of the ball. I had to defend a lot," Murray said in an on-court interview. "I think he played some really, really good tennis. It made for an entertaining match."

After pulling out the first set in a tiebreaker, Djokovic needed less than an hour to close out his victory. The 2011 champion beat 87th-ranked Benjamin Becker of Germany 7-6 (2), 6-2, 6-2.

Becker had a chance to serve for the first set against Djokovic at 5-4. But he wasted the first set point with a forehand into the net and the second when a backhand sailed wide.

"Becker is a quality opponent and he should have won the first set," Djokovic said. "I was fortunate enough to come back and win the first set, and after that, I was much more comfortable on the court."

Djokovic was playing in Arthur Ashe Stadium, where the wind often swirls to players' frustration.

"You need to have this adjustment, footwork, steps, in order to get on the ball," Djokovic said. "I didn't have that in the first set. I was still trying to find my way from baseline, and the other side he was serving well. ... After I played a good tiebreak, everything kind of settled. I started to serve better, started to step into the court, which is important."

Murray was in Louis Armstrong Stadium, a place that had bedeviled him in the past. Last year, he was pushed to four tough sets in the third round and quarterfinals there.

Another Grand Slam winner, Li Na, also showed some championship form on Ashe. This time, her opponent, Laura Robson, looked very much like a 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 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.

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."

Australian Open semifinalist Sloane Stephens, seeded 15th, beat fellow American Jamie Hampton 6-1, 6-3. Hampton, at No. 23, was seeded at a Grand Slam tournament for the first time in her career.

Stephens' next opponent could be defending champ Serena Williams, who faces Yaroslava Shvedova in the nightcap at Ashe. It would be a rematch of their Australian Open quarterfinal, won by the young American.

Third-seeded Agnieszka Radwanska, No. 9-seeded Angelique Kerber and No. 9 Jelena Jankovic all advanced in straight sets.

Wimbledon runner-up Sabine Lisicki, seeded 16th, won't be making a run at Flushing Meadows after she was eliminated in straight sets by Ekaterina Makarova. The 24th-seeded Russian won 6-4, 7-5.

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.

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.

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 teens: 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."