True -- but the same argument can be made for Graf, Evert, and Court as well. When it comes to doubles, Martina is the GOAT. But usually in these discussions people are talking about the singles GOAT, and doubles only comes into play when trying to separate similarly rated singles players.
Most hardcourt grand slam singles titles
By winning the 2001 Australian Open doubles championship with Venus Williams, became the fifth pair to complete a Career Doubles Grand Slam and the only pair to win a Career Doubles Golden Slam.
By winning the 2005 Australian Open by defeating Mauresmo and Davenport, became the only player in tennis history to win three Grand Slam singles titles (1999 U.S. Open, 2002 Roland Garros) by beating the top two ranked players.
Only player to have won three Grand Slam singles titles after saving match points (2003 Australian Open versus Kim Clijsters, 2005 Australian Open versus Maria Sharapova, and 2009 Wimbledon versus Elena Dementieva).
Lowest ranked player (No. 18) to defeat the Top-2 players in the world in the same tournament
She became the only person in history, male or female, to win three of the four grand slams 6 times
Only player, male or female, to complete the Career Golden Slam after turning 30 years of age
Only player, male or female, to win 10+ grand slam singles titles in two separate decades (10 in the 2000s and 12 in the 2010s)
By reaching the final of the 2003 Australian Open, she and sister Venus became the first players to compete in 4 consecutive slam finals
The only pair to win a Career Doubles Golden Slam and first pair in the Open Era to complete the Career Doubles Golden Slam twice
Set a record for most aces served in a tournament, hitting 102 aces in the tournament, which was the most of men or women at the Championships that year
Longest span between first (2003) and last (2017) Australian Open titles
Longest span between first (2002) and last (2015) Roland Garros titles
Longest span between first (2002) and last (2016) Wimbledon titles
Longest span between first (1999) and last (2014) US Open titles
Longest-ever gap between stints at No. 1 (five years, one month)
First woman to win 60+ career matches in all four Grand Slams in the Open Era
Oldest player to hold all Grand Slam singles titles simultaneously
316 grand slam singles match wins is more than any player, male or female, in tennis history
First, and only, woman to win US$80,000,000 in career prize money
Highest earning female athlete of all-time in terms of prize money across all sports
Most decorated tennis player in the Olympics with a record of 4 gold medals shared with sister, Venus.
Oldest player to hold No. 1 ranking
1st player to have more than twice as many points as anyone else on the WTA Rankings
Oldest player to win a grand slam singles title
First player in tennis history to win the Season-Ending Championships on her debut
First woman in history to win singles titles at (1) a Grand Slam event, (2) a Premier Mandatory event, (3) the Olympics [gold medal], and (4) the WTA Championships in the same season
Only player, male or female, to ever accomplish a Career Golden Slam in singles and doubles
Destroying revisionist haters is so fun!