Early years

Stevenson made her professional tennis debut in 1998 as a wild card at the US Open where she lost in the first round to France's Alexandra Fusai. In 1999 Stevenson qualified and ultimately reached the quarterfinals at Birmingham, a grass warmup to Wimbledon, before withdrawing to Magüi Serna because of a stomach muscle pull. The next week, Stevenson was the number one seed during qualifying at Roehampton—and did not drop a set in three rounds as she moved into the main draw at Wimbledon.

At Wimbledon, two weeks after graduating from La Jolla Country Day School, she became the second woman qualifier in the tennis Open Era to reach the semifinals. It was her second time at Wimbledon, having competed the year before in the Junior Wimbledon Championships. Stevenson beat 11th seed Julie Halard in the third round. In the fourth round, she saved one match point against Lisa Raymond in a 2–6, 7–6, 6–1 win. She then beat Jelena Dokić in three sets in the quarterfinals, before losing to the eventual champion, third seed Lindsay Davenport. Stevenson had 57 aces during the fortnight, her serve speed over 120 mph. It was the beginning of Stevenson being known for the fastest second serve in the women's game. Stevenson's second serve was 105 mph – 115 mph. Phil Knight, the co-founder and chairman of Nike, flew to London to personally sign Stevenson to a three-year contract.

During the months following her dramatic run at Wimbledon, Stevenson was named Rookie of the Year by Tennis Magazine and named Most Fascinating by People Magazine. She was interviewed by Barbara Walters for a Barbara Walters Special and was featured in a variety of national and international media. Nike flew a dozen flags with Stevenson's name in bold print during her first visit to the iconic campus. Stevenson moved from California to train with Nick Bollettieri at IMG Academy from 1999 to 2002. In 2000 and 2001, Stevenson, often pitted against top 20 players – including Nathalie Tauziat, Mary Pierce, Martina Hingis, Monica Seles, Amanda Coetzer, Lisa Raymond, Dominique Van Roost, Julie Halard, Jennifer Capriati, Serena Williams, and Venus Williams – worked on her aggressive all-court playing style as she found her way in the professional game.

Stevenson reached the quarterfinals at Quebec City in 2001, and the quarterfinals at Linz, defeating Arantxa Sánchez Vicario en route.

But, it was in 2002 that Stevenson moved up to No. 18 in the world. She had played during the European indoor season in 2001 and was the only American player to appear in Moscow in 2001 following the September 11 terrorist attacks. In 2002, prior to the European indoors, she lost to Monica Seles in the Tokyo quarterfinals 6–7⁽⁹⁾, 6–7⁽⁹⁾, and there were no breaks of serve until the tiebreakers. Alan Mills, the Tokyo referee and renowned Wimbledon referee proclaimed, "This is the finest women's serving match I have ever seen." Soon after, Seles and Stevenson were asked to serve aces for the American Heart Association, bringing awareness to heart disease. Stevenson served 100 aces in 2003. During the 2002 European indoors, Stevenson won her first doubles title with Serena Williams in Leipzig, Germany. She reached the quarterfinals of Filderstadt, defeating number one Jennifer Capriati. She was a finalist at Linz, beating two top ten players en route, including Capriati. Stevenson finished the year at 18, percentage points away from No. 17, Elena Dementieva.

Injury and return

Stevenson experienced shoulder pain in 2003 at the Wimbledon Championships. She worked on strengthening the area, but was unable to repair the tear. Stevenson went to Birmingham, Alabama, where Dr. James Andrews performed a type II labral repair on her right shoulder in September 2004. In 2006, following 18 months of shoulder strengthening, Stevenson began to mount a comeback.

Stevenson played in the qualifying round at 2006 Wimbledon, but after defeating Jelena Dokić, a player that she had played in 1999 in the Wimbledon quarterfinals, when they were both qualifiers, she suffered a pectoral strain and fell in the second round of qualifiers. At the 2006 Cincinnati Open, she also had a successful run in the qualifying round, but continued shoulder pain caused her to fall in the final round to Chan Chin-wei, in three sets. By 2009, Stevenson gained shoulder strength and posted strong results. In 2010, Kevin Wilk, Dr. Andrews' physical therapist, said that Stevenson's shoulder "feels like a non-surgical arm."

Stevenson won the Sarasota Clay Court Invitational in April 2012.

Playing style

Stevenson joined Venus Williams, Serena Williams, Lindsay Davenport, Monica Seles, Mary Pierce and Jennifer Capriati as a premier power player on the WTA Tour in 1999. 6' 1" and right-handed, she plays with a one-handed backhand. Her serve, forehand, and one-handed backhand are noted weapons in Stevenson's all-court game. Her fastest serve was clocked at 125 mph. She had the fastest second serve in the women's game from 1999 to 2004 at 105–115 mph. She was the first woman to amass 57 aces during the Wimbledon fortnight in 1999. The power game came from years of repetitive lessons. At nine years old, Stevenson began traveling from her home in San Diego to Los Angeles to be coached by Robert Lansdorp and Pete Fischer. It was Lansdorp who developed her powerful ground game, changing her two-handed backhand to a one-handed backhand. Lansdorp would tie her arm with an ace bandage to work on the backhand motion. Fischer, who also coached Pete Sampras, developed Stevenson's service motion, often used by coaches to teach "the perfect service motion." Fischer designed service drills to resemble Sampras' fluid serve.

Broadcast career

Starting in August 2019, Stevenson joined the ESPN tennis commentating team in their coverage of the qualifying and main draws of the 2019 US Open. She also did the commentating at the 2020 and 2021 US Open for ESPN.

Singles: 2 (2 runner-ups)

Result	W–L	Date	Tournament	Tier	Surface	Opponent	Score
Loss	0–1	Feb 2002	U.S. National Indoor Championships, United States	Tier III	Hard (i)	Lisa Raymond	6–4, 3–6, 6–7(9–11)
Loss	0–2	Oct 2002	Linz Open, Austria	Tier II	Hard (i)	Justine Henin	3–6, 0–6

Doubles: 1 (title)

Result	W–L	Date	Tournament	Tier	Surface	Partner	Opponents	Score
Win	1–0	Sep 2002	Sparkassen Cup Leipzig, Germany	Tier II	Carpet (i)	Serena Williams	Janette Husárová Paola Suárez	6–3, 7–5

Singles: 2 (1 title, 1 runner–up)

Result	W–L	Date	Tournament	Tier	Surface	Opponent	Score
Win	1–0	Feb 1998	Midland Tennis Classic, United States	50,000	Hard (i)	Samantha Reeves	7–6(10–8), 6–1
Loss	1–1	May 2009	Carson Challenger, United States	50,000	Hard	Valérie Tétreault	6–4, 2–6, 4–6

Doubles: 2 (2 runner-ups)

Result	W–L	Date	Tournament	Tier	Surface	Partner	Opponents	Score
Loss	0–1	May 2015	ITF Indian Harbour Beach, United States	50,000	Clay	Angelina Gabueva	Maria Sanchez Taylor Townsend	0–6, 1–6
Loss	0–2	Sep 2016	Atlanta Tennis Open, United States	50,000	Hard	Taylor Townsend	Ingrid Neel Luisa Stefani	6–4, 4–6, [5–10]