quality:corruption
2008–2010: Injuries, controversy, and return to No. 1

Williams started 2008 by winning the Hopman Cup for the US, with Mardy Fish. At the Australian Open, she lost in the quarterfinals to Janković, her fourth straight loss in the quarterfinals of a Grand Slam singles tournament. In the women's doubles event, she and Venus were defeated in the quarterfinals. Williams withdrew from her next three scheduled tournaments because of an urgent need for dental surgery. She then won three consecutive singles titles at Bangalore and her fifth Miami title, tying Graf for the most singles titles at this tournament. She claimed victory at the Family Circle Cup, her first clay-court title since 2002. Her 17-match winning streak was ended by Dinara Safina in the quarterfinals in Berlin. Williams withdrew in Rome in the quarterfinals due to a back injury, and lost in the third round of the French Open to Katarina Srebotnik.

At Wimbledon, Williams reached the finals for the first time in four years, but lost to Venus in their first Grand Slam final since 2003. Serena and Venus teamed up and won the doubles title, however. Williams played at Stanford, but retired in the semifinals with a knee injury, which also forced her to withdraw from a tournament in Los Angeles. During the Summer Olympics in Beijing, Williams lost to Dementieva in the singles quarterfinals, but she and Venus won the gold medal in doubles. In early September, Williams captured her third US Open title, which was also her ninth Grand Slam singles title. The victory returned her to No.1 for the first time since 2003. At the year-end championships, she defeated Safina and lost to Venus in round-robin matches, but withdrew from a match against Dementieva, citing a stomach muscle injury. She ended 2008 ranked No.2 and with four singles titles, her strongest performance in both respects since 2003.

Williams began 2009 at the Medibank International, losing in the semifinals to Dementieva. At the Australian Open, she claimed her tenth Grand Slam singles title by defeating Safina. This win restored her No.1 ranking and made her the all-time career prize money leader in women's sports, a title previously held by golfer Annika Sörenstam. Serena and Venus also won the doubles event at the Australian Open for the third time. At the Open GdF Suez, Williams withdrew before her semifinal match against Dementieva because of a knee injury. She then played in Dubai, losing to Venus in the semifinals.

Williams was beset by ankle and quadriceps injuries at the Sony Ericsson Open, and was upset in the final by Victoria Azarenka. This was the first of four consecutive losses for Williams, the longest losing streak of her career. After Ericsson, she was defeated in her opening matches in Barcelona, Rome, and Madrid. At the French Open, she lost in the quarterfinals to the eventual champion Kuznetsova. This ended her 18-match winning streak at Grand Slam tournaments. She rebounded at Wimbledon, prevailing over Dementieva in the semifinals and Venus in the finals. The victory was her third Wimbledon title and her 11th Grand Slam singles title. For the second consecutive year, Serena and Venus claimed the Wimbledon doubles title, which was their ninth Grand Slam title in doubles.

Ahead of the US Open, Williams suffered a third-round defeat in Cincinnati and a semifinal defeat at the Rogers Cup. At the Open, she was given a racket abuse warning after losing the first set of her semifinal match against Clijsters. Later in the match, one of her serves was called a foot fault. Williams yelled profanities at the lineswoman who made the call, and threatened to shove a tennis ball down her throat. Williams was penalized a point for unsportsmanlike conduct, which resulted in Clijsters winning the match. The following day, Williams was issued a fine of $10,500. After further investigation, the Grand Slam Committee fined her $175,000 in place of suspending her from the 2010 US Open or other Grand Slams. They also placed her on a two-year probation, which meant that if she committed another offense at a Grand Slam during the next two years, she would be suspended from the following US Open. If, however, she committed no offenses, her fine would be reduced to $82,500. Although Williams initially did not express regret for her outburst, she eventually apologized, saying she was humbled by the experience.

Williams continued in the US Open doubles competition, teaming up with Venus to capture their third Grand Slam doubles title of the year, and the tenth of their career. Williams won all three of her round-robin matches at the year-end WTA Tour Championships, defeating Venus, Dementieva, and Kuznetsova. She advanced to the final after Caroline Wozniacki retired from their semifinal match. In the final, Williams defeated her sister to claim her second singles title at this event. Williams finished the year ranked No.1 for the second time in her career. She played in 16 tournaments in 2009, more than any other year. With $6,545,586 in prize-money earnings, she broke the record previously set by Justine Henin for most prize money earned by a female tennis player in one year. In doubles, the Williams sisters finished 2009 at No.2, despite playing only six tournaments together. Williams now had a total of 23 Grand Slam titles, and was consequently named Female Athlete of the Year by the Associated Press. She was also the International Tennis Federation World Champion in both singles and doubles.

In 2010, Williams's first tournament was in Sydney, where she lost in the final to Dementieva. At the Australian Open, Williams was the defending champion in both singles and doubles. She reached the final and defeated Justine Henin, who had recently come out of retirement, for her twelfth Grand Slam singles title. In doubles, Williams and her sister successfully defended their title by defeating Cara Black and Liezel Huber in the final. Williams sat out several events with a leg injury, but returned for the Rome Masters, where she was defeated by Janković in the semifinals. At Madrid, she fell to Nadia Petrova in the third round, but partnered with Venus to win the doubles title.

At the French Open, Williams was bested by Samantha Stosur in the quarterfinals. She and Venus won the doubles event, achieving their fourth consecutive Grand Slam doubles title and improving their doubles ranking to No.1. Williams's next tournament was Wimbledon, where she did not lose a single set, defeating Zvonareva in the final. After the match, Navratilova said Williams was among the top five female tennis players in history. She asserted that being a great player is "not just about how many Slams you win... it's just your game overall... she's got all the goods." The Williams sisters lost in the doubles quarterfinals to Elena Vesnina and Zvonareva. In Munich on July 7, Williams stepped on broken glass while in a restaurant, and the injury caused her to miss the rest of the year. She finished 2010 ranked No.4 in singles and No.11 in doubles. On March 2, 2011, she confirmed that she had suffered a hematoma and a pulmonary embolism.