It took Williams seven match points to finally get the job done, leaving the tournament favorite visibly and audibly frustrated. Despite her struggles to end the match, though, Serena ultimately disposed of Li, who was making her first semfinal appearance at the U.S. Open.
Williams didn't end the match as brilliantly as she started it, but the result of the contest was hardly ever in question. Peppering the hard surface with triple-digit serves and powerful backhands, Serena was in control for the vast majority of the semifinal. She won just over three quarters of her first-serve points and had 19 winners.
Coming off a 6-0, 6-0 drubbing of Carla Suarez Navarro in the quarterfinals, Williams made it clear early on that she had plans for another dominant effort. She opened the match by blanking Li in the first game, and broke the Chinese star with startling ease in her first game on serve.
The pace of the first frame played into Williams' hands. With a series of hard-struck, two-handed shots, Serena sent the smaller Li scurrying across the court to save points. The result was a series of mishits, as Li had just one winner against 12 unforced errors in the opening frame.
Williams' overpowering nature allowed her to win 85 percent of her first-serve points, helping to atone for nine unforced errors of her own.
Serena won her opening service game of the second set thanks to more errors from Li, marking the 24th consecutive that she had won dating back to her fourth-round match against Sloane Stephens.
However, just as soon as talk of the streak became the prevailing subject of conversation, Li made her only run of the match. The 2011 French Open champion held serve for the first time, and then broke a frustrated Williams to take a 2-1 lead.
Ahead 40-0 on her subsequent service game with a chance to go ahead 3-1, the momentum appeared to be shifting in Li's direction. But the powerful American roared back, with Serena winning the next three points and ultimately evening the set back up at 2-2.
At that point, the match was ostensibly over. Williams took the next three games from a gassed Li, putting the reigning champ on the precipice of closing out the match with ease.
Li would not go down without a fight, though, holding off six break points in the second set's eighth game, each time watching Williams become more outwardly flustered. But as had been the case throughout the match, Serena's sheer power put an end to Li's late surge, ending the match at one hour and 26 minutes.
Williams, looking for her fifth U.S. Open title, has been in championship form since the event began. She's yet to give up more than four games in one set, and has been particularly impressive in close-out sets, having lost a total of only five games through six matches.
Next up for Serena is second-seeded Victoria Azarenka in a rematch of last year's final at Flushing Meadows. Azarenka defeated Flavia Pennetta in Friday's first semifinal—a hard-fought match that saw only five games with a held serve.
Who will win women's final at 2013 U.S. Open?
The 24-year-old Belarusian lost to Williams 6-2, 2-6, 7-5 last year in a scintillating three-set contest. Although Williams will likely be favored in Sunday's match, Azarenka has plenty of reason to feel optimistic about her chances. She's a WTA-best 31-1 on hard surfaces this season, having won each of the past two Australian Opens. She also defeated Williams in the 2013 Western & Southern Open final in three sets.
With that said, Williams holds a 12-3 career record against her second-ranked foe, and considering how she has played through the U.S. Open thus far, it will take an incredible effort from Azarenka to come away with the win.
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