It's tough, but beating Serena Williams at a major is possible

Updated 1120 GMT (1920 HKT) September 10, 2015
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Serena Williams has been dominant at the majors, especially since last year's U.S. Open. But she has been beaten before at grand slams ... Mike Stobe/Getty Images for the USTA
The only time Williams lost in the first round of a grand slam came in 2012 against Frenchwoman Virginie Razzano at the French Open. PATRICK KOVARIK/AFP/AFP/Getty Images
Razzano, whose fiance died a year earlier, rallied from a set down and 5-1 down in a second-set tiebreak to pull off the shock win. PATRICK KOVARIK/AFP/AFP/Getty Images
The last player, meanwhile, to beat Williams in a grand slam final was Samantha Stosur at the 2011 U.S. Open. Al Bello/Getty Images North America/Getty Images
Williams coming second in a grand slam final is rare: Her record in major finals stands at 21-4. Mike Stobe/Getty Images North America/Getty Images for USTA
Stosur is one of three players who have played Williams at least twice at a grand slam to have a winning record against the world No. 1. The other two are Justine Henin, seen here, and Jennifer Capriati. JOE KLAMAR/AFP/AFP/Getty Images
One of Capriati's four wins over Williams took place at the US Open in 2004, when the latter was on the receiving end of at least one bad officiating call. DON EMMERT/AFP/AFP/Getty Images
Who was the last player to beat Williams at a grand slam? It's Razzano's countrywoman, Alize Cornet. She did it last year at Wimbledon. Steve Bardens/Getty Images Europe/Getty Images
Garbine Muguruza toppled Williams at the 2014 French Open but Williams gained her revenge in July, defeating the Spaniard in the Wimbledon final. Matthew Stockman/Getty Images Europe/Getty Images
Williams' defeats, however, are rare, and she will be expected to complete the calendar-year grand slam in September. She's the three-time defending champion at the U.S. Open. Chris Trotman/Getty Images North America/Getty Images for USTA