But then the list of players Williams has lost to in 2013 is short -- Sloane Stephens at the Australian Open quarterfinals, Victoria Azarenka in the finals at Doha, Sabine Lisicki in the fourth round at Wimbledon and Azarenka in the finals in Cincinnati.
Williams is 60-4 this year with eight tournament titles, including a win at French Open for her 16th career Grand Slam win. Only three players -- Steffi Graf with 22 and Chris Evert and Martina Navratilova both with 18 -- have more.
She returned to the top of the women's tennis rankings in February.
This week Williams begins the battle for a fifth U.S. Open championship. She won her first major tournament in New York in 1999 and took the U.S. Open in 2002 and '08. She beat Azarenka 6-2, 2-6, 7-5 in winning the 2012 final.
Given the way she has dominated the WTA this year, Williams is a strong favorite to get another Grand Slam title when the women's singles is decided Sept. 8.
Azarenka is the main challenger. No. 2 in the world and having handed Williams half her losses this year, Azarenka is 36-4 (one loss to Williams) with three titles. She won the Australian Open early this year but last year's trip to the finals was the only time in seven tries she advanced past the fourth round in New York.
With No. 3-ranked Maria Sharapova out of the U.S. Open with a shoulder injury, the bulk of the attention will be on Williams and Azarenka.
Some names that could crop up later in the tournament, however, include Sara Errani and Roberta Vinci, world No. 1 doubles partners who have had solid singles seasons and are seeded fourth and 10th, respectively. Simona Halep, seeded 21st, has the second most titles this year (four) and seems to be pushing her game along nicely.
Fourth-ranked Agnieszka Radwanska has a pair of championships in 2013 but is 4-8 against Top 10 players, a figure she'd likely have to improve to have a chance in New York.
Stephens is 29-18 this year but has that signature win over Williams and, with strong backing from the U.S. fans, could surprise and run deep into the tournament. She is yet to win a WTA event, so she is a long shot to get that first championship at a Grand Slam event.