Victoria Azarenka struggled in her semifinal win over Flavia Pennetta and will have to be much sharper if she hopes to beat Serena Williams in Sunday’s women’s final at the U.S. Open. (David Goldman/AP)

Serena Williams has steamrolled to a staggering 65-4 record this year. And only one player has beaten her more than once, Victoria Azarenka, who boasts the power to fend off Williams’s blasts and a temperament that craves huge tests rather than cowering in the face of them.

But if Friday’s semifinals at the U.S. Open proved anything, it’s that Azarenka, a hard-hitting 24-year-old from Belarus, must improve considerably in the next 48 hours if she’s to have a prayer against Williams in Sunday’s reprise of their final last year.

The world’s No. 2 player, Azarenka overcame nerves and a shaky serve to clinch her spot in Sunday’s final with a 6-4, 6-2 victory, but only because her opponent, unseeded Flavia Pennetta, had an even worse serving day, getting broken eight times out of nine service games.

The match was a case study in how to win without playing particularly well and, in stretches, a testament to the perils of mental frailty.

In the opening set, Azarenka won the point less than half the time on her first serve, got broken three times and committed more than twice as many unforced errors (18) and winners (eight). But she still won the set because Pennetta, appearing in the semifinal of major for the first time, was even shakier.

“We didn’t serve really well, both of us,” Pennetta said. “But in the end, she’s powerful player. She have more power than me.”

Williams, by contrast, hasn’t been tested all tournament.

Friday’s matchup with fifth-seeded Li Na held the promise of a battle. Li, the 2011 French Open champion, has dramatically retooled her game since firing her husband as her coach and hiring Carlos Rodriguez, who led the undersized Justine Henin to seven major titles during her career.

In the past 13 months with Rodriguez, Li, 31, has improved her foot speed, strengthened her serve, honed her volleys and adopted an attacking style of play that has served her well through the tournament’s first five rounds, in which she lost only one set.

But she was rendered powerless by Williams, who won the first seven games to sail into Sunday’s final with a 6-0, 6-3 victory.

Though she has played big matches before, reaching the finals of the Australian Open this year, Li said she was overwhelmed upon walking out on Arthur Ashe Stadium on Friday.

“Should not be nervous because not the first time to play semis,” said Li, the first Chinese player to reach the U.S. Open’s semifinals. “But when I walk to the court, was feeling the court so big. . . . I was feeling like football court.”

Li finally held serve in the second game of the second set, then broke Williams to take a 2-1 lead. It was short-lived. Williams countered with tremendous defense and reclaimed the momentum.

All the drama in the match was packed into Li’s must-hold service game at 2-5 in the second set. It was a nearly 14-minute ordeal, in which Li fended off six match points before holding when Williams blasted a forehand long.

After letting out a howl of frustration, Williams collected herself to close the match in the next game.

“It got tough at the end,” Williams said during her on-court interview with CBS. “I got a little nervous, but I was able to close it out finally.”

The 6-0, 6-3 rout came on the heels of Williams’s 6-0, 6-0 dismantling of 18th seeded Carla Suarez Navarro in the quarterfinals.

The upshot puts Williams one victory from her fifth U.S. Open title and her 17th major overall, which would pull her within one of tying the mark of 18 majors shared by Chris Evert and Martina Navratilova.

Azarenka’s 3-12 record against Williams suggests Sunday’s final won’t be much of a fight. But the Belarusian has won two of their last three encounters, including both hard-court clashes this year, in tough three-setters.

Azarenka served for the U.S. Open title last year, leading Williams 5-3 in the third set only to lose focus, the next four games and the match.

“I’m a better player,” Azarenka said, asked about last year’s defeat. “I’m a more complete player.”

And she saw little point in revisiting that match as a means of preparing for Sunday’s final, which pits the world’s No. 1 player against No. 2.

“Not that I don’t care what happened in the past, but I think there is no need for paying so much attention to what happened,” Azarenka said. “It’s always a new story.”

A two-time Australian Open champion, Azarenka has not yet shown her top form in New York. As Azarenka searches for it, having twice been pushed to three sets, Williams, arguably the greatest women’s player in the sport’s history, is playing the best tennis of her career on the eve of her 32nd birthday.

Note: Less than two hours after her semifinal victory over Li, Williams returned to Ashe to contest the women’s doubles semifinal with her sister Venus. They were defeated by the Czech duo of Andrea Hlavackova and Lucie Hradecka, 6-4, 6-2.