Serena is the third oldest female Grand Slam finalist in the Open era at 36 years and 291 days.
Serena Williams made a winning return after more than a year away from tennis by beating Zarina Diyas 7-5, 6-3 to advance on Thursday at the Indian Wells Masters.
Serena Williams returned to the top of the WTA world rankings on Monday and immediately shared the news with her unborn baby.
Serena Williams beat Germany's Angelique Kerber 7-5 6-3 to claim a seventh Wimbledon singles title and equal Steffi Graf's professional era record of 22 grand slam singles crowns on Saturday.
Angelique Kerber booked a Wimbledon final showdown with defending champion Serena Williams as the German ended Venus Williams' bid for a fairytale title with a 6-4, 6-4 victory on Thursday.
Indian Wells tournament director Raymond Moore quit under heavy fire after he said women players owe their success to men, sparking a furious gender row that has divided the sport.
World number one Serena Williams ripped "offensive" remarks by Indian Wells tournament director Raymond Moore, who claimed women's tennis was riding on the coattails of the men's game.