Serena Williams faces Na Li on Friday in the 2013 US Open semifinals.  The match is scheduled for 3:45 p.m. ET.  Watch the live stream HERE or tune to CBS.

World No. 1 Williams has been unstoppable for most of the season.  She's carried her hot streak in to the US Open, where she blew through her first five opponents without dropping one set.  Even more impressive, Williams has lost just 13 games through the first five rounds of the Open, according to NY Daily News.

Williams, 31, admitted before the tournament that everything she accomplishes now in her career is a bonus.  Despite her view, she isn't playing like everything someone content with her collection of 54 WTA singles titles.

She isn't playing like her age, either.

"Mentally, she's different.  She really wants it," Patrick Mouratoglou, Williams' coach since last June, told The News.  "She's working like she's 19 years old and wants to win her first Grand Slam.  She has the same appetite.

"That makes a huge difference.  How many players being (almost) 32 have the same appetite and especially after winning so many Grand Slams?  I mean, they are different.  Serena is really different, mentally."

To continue the quest for her fifth US Open title and earn a spot in the final, Williams must battle past Li.

Li, 31, is ranked No. 6 in the World with a 37-11 record this year and the Shenzhen Open title to her name.  She's played well to earn a spot in the semifinals - her only dropped set of the tournament was in the quarterfinals against Ekaterina Makarova.

In 10 career matches against Williams, Li has only won two.  They last faced each other this year at Cincinnati in the semifinals, where Williams won in two sets.  

*Stats via WTATennis.com and MatchStat.com.