Question: Why is Serena Williams more accomplished than Mariaan de Swardt in her tennis career?

Passages:
The 1999 Wimbledon Championships – Women's Doubles was the women's doubles event of the hundred-and-thirteenth edition of Wimbledon, the third Grand Slam of the year, the most prestigious tournament on grass courts, and the oldest tennis tournament in the world.  Martina Hingis and Jana Novotná were the defending champions but only Novotná competed that year with Natasha Zvereva.  Novotná and Zvereva lost in the semifinals to Mariaan de Swardt and Elena Tatarkova. Gigi Fernández and Natasha Zvereva were the defending champions and won in the final 7–6, 6–3 against Mariaan de Swardt and Irina Spîrlea. Mariaan de Swardt and David Adams were the defending champions but only Adams competed that year with Kristie Boogert. Mariaan de Swardt won in the final 3–6, 7–6, 7–5 against Barbara Schett. Lisa Raymond and Rennae Stubbs won in the final 6–4, 6–4 against Mariaan de Swardt and Mary Joe Fernández. Katrina Adams and Mariaan de Swardt were the defending champions but did not compete that year. Debbie Graham and Brenda Schultz-McCarthy were the defending champions, but decided not to compete together.  Graham partnered with Mariaan de Swardt, but lost in the semifinals to Alexandra Fusai and Nathalie Tauziat.  Schultz-McCarthy partnered with Rebecca Jensen, but lost in the first round to Patricia Hy-Boulais and Chanda Rubin. The Williams sisters are two professional American tennis players: Venus Williams (b. 1980), a seven-time Grand Slam title winner (singles), and Serena Williams (b. 1981), twenty-three-time Grand Slam title winner (singles), both of whom were coached from an early age by their parents Richard Williams and Oracene Price.  There is a noted professional rivalry between them – between the 2001 US Open and the 2017 Australian Open tournaments, they met in nine Grand Slam singles finals.  They became the first two players, female or male, to play in 4 consecutive grand slam singles finals from the 2002 French Open to the 2003 Australian Open; Serena famously won all 4 to complete the first of two "Serena Slams".  Between 2000 and 2016, a 17-year span, they collectively won 12 Wimbledon singles titles (Venus won 5 and Serena won 7).  By winning the 2001 Australian Open women's doubles title, they became the 5th pair to complete the Career Doubles Grand Slam and the only pair to complete the Career Doubles Golden Slam.  At the time, Venus and Serena were only 20 and 19 years old, respectively.  Since then they have gone on to add another two Olympic gold medals in the 2008 Beijing Olympics and the 2012 London Olympics.  Nearly a decade later, the duo would go on to win 4 consecutive grand slam doubles titles from 2009 Wimbledon through 2010 Roland Garros, which would catapult them to co-No.  1 doubles players on 7 June 2010.  Two weeks later, on 21 June 2010, Serena would hold the No. 1 singles ranking and Venus would be right behind her at No. 2 in singles.  Their most recent grand slam doubles titles came at the 2012 Wimbledon & 2016 Wimbledon events.  They remain very close, often watching each other's matches in support, even after one of them has been knocked out of a tournament. Mariaan de Swardt (born 18 March 1971) is a former tennis player from South Africa, who played as a professional from 1988 to 2001.  She twice represented her native country at the Summer Olympics, in 1992 and 1996, and was a member of the South African Fed Cup Team in 1992 and 1994–1997.  In 2006, de Swardt became a U.S. citizen. Serena Jameka Williams (born September 26, 1981) is an American professional tennis player.  The Women's Tennis Association (WTA) has ranked her world No. 1 in singles on eight occasions, from 2002 to 2017.  She became the world No. 1 for the first time on July 8, 2002.  On the sixth occasion, she held the ranking for 186 consecutive weeks, tying the record set by Steffi Graf for the most consecutive weeks as world No. 1 by a female tennis player.  In total, she has been world No. 1 for 319 weeks, which ranks her third in the Open Era among female tennis players.  Some commentators, players and sports writers regard her as the greatest female tennis player of all time.

Supporting sentences:
Serena Jameka Williams (born September 26, 1981) is an American professional tennis player.  She became the world No. 1 for the first time on July 8, 2002. Mariaan de Swardt (born 18 March 1971) is a former tennis player from South Africa, who played as a professional from 1988 to 2001.  She twice represented her native country at the Summer Olympics, in 1992 and 1996, and was a member of the South African Fed Cup Team in 1992 and 1994–1997.