Top 6 moments that show why Serena Williams is the GOAT in women's tennis

Serena Williams played what could be the last-ever match of a glorious career as she went down fighting against Ajla Tomljanović in the third round of the US Open on Friday.

Williams, who had earlier announced that she would "evolve away" away from the game, refused to reconsider her decision to hang up her racquet but did not rule out a comeback, saying "you never know" when quizzed about her plans.

The American legend dominated the women's game like none of her predecessors ever did and displayed, by virtue of her sheer dominance, an array of records, and ability to win on all surfaces, that she is arguably the Greatest Of All Time (GOAT).

Let's take a look at six defining moments (in no particular order) that make a case for why Serena Williams was the GOAT in women's tennis.

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#6. Serena Williams is the first player to win titles in four separate decades (1999-2020)

After winning her 23rd and final Grand Slam title while pregnant, Serena Williams made a comeback after becoming a mother.

Despite overcoming near-fatal complications during childbirth, she returned to action and reached a couple of Grand Slam finals as well. She lost to Angelique Keber in the finals of the 2018 Wimbledon Championships and to Naomi Osaka in the finals of the US Open the same year.

Having dropped to 491st in the rankings, she returned to the top-10 in February 2019 and went on to win the 2020 ASB Classic by defeating Jessica Pegula. Thanks to that win in Auckland, the tennis legend became the first player in history to win a title in four separate decades.

Williams won her first professional title when she got past Amelie Mauresmo in the final of the Open Gaz de France in 1999 before going on to win the French Open and the US Open that year.

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#5. Taking a stand to boycott the Indian Wells Open for 14 years

Naomi Osaka, who was in tears after being hassled by a heckler at the Indian Wells Open earlier this year, revealed that the incident reminded her of what Serena Williams had undergone in 2001.

Back then, the Williams family became the target of slurs and racial abuse after Venus Williams withdrew from her semifinal encounter against her sister, which led to accusations of match mixing.

Russian player Elena Dementieva also accused their father Richard Williams of attempting to manipulate the tournament.

The sisters decided to take a strong stand and boycotted the Indian Wells Open for 14 years to send out a strong message before returning to compete in 2015.

Williams' resolve in standing up for what she believed complemented her on-court prowess.

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#4. Overcoming depression to win 2007 Australian Open

Despite Serena Williams' emergence as a force to reckon with early in her career, it wasn't all smooth sailing for the champion.

Off-court events like the murder of half-sister Yetunde Price took a toll on the youngster, which led to a slump in her form from the second half of 2003 up until 2007. She later revealed that she also suffered from bouts of depression in 2006 before entering the 2007 Australian Open as an unseeded contestant.

Battling the weight of expectations and the pressure of being served an ultimatum by her sponsor Nike, Williams advanced to the finals despite developing blisters on her feet en route.

World No. 1 Maria Sharapova loomed in the title round, but Serena Williams stunned the tennis world by winning the tournament, which was just the beginning of a phenomenal return to the top.

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#3. Second "Serena slam" in 2015

After becoming the first player in history to win the Golden Slam in singles and doubles in 2012, Serena Williams did the unthinkable by adding a sixth Wimbledon title to her kitty in 2015 whereby she achieved a second "Serena Slam."

She won the US Open in 2014, easing past Caroline Wozniacki in straight sets, before reigning supreme at the Australian Open in 2015 where she accounted for Maria Sharapova.

Lucie Safarova managed to take the second set in the 2015 finals at Roland Garros but failed to prevent Williams from winning her third French Open title, while Garbine Muguruza could do little to prevent the American from creating history.

Four Grand Slam wins in a row and a 21st overall Major title to draw level with Steffi Graf's tally had guaranteed Williams a place as one of the greatest players ever to have picked up a racquet.

Back in 2012, Serena Williams won the Olympic gold medal by beating Maria Sharapova, who was chasing her own Golden Slam after having achieved a career Grand Slam by winning the French Open earlier that year.

Partnering sister Venus, she also won gold in the women's doubles competition in London 2012. Williams had already achieved a career Grand Slam as early as 2003 by winning the Australian Open.

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#2. Winning a 23rd Grand Slam while still pregnant

While not many knew at the time, Serena Williams won her 23rd Grand Slam title at Melbourne Park in 2017 while she was pregnant.

The remarkable title win, achieved by not dropping a set, was astonishing in itself given that she was 35 years old at the time. The revelation that she was pregnant added a whole new dimension to the significance of the triumph.

When Williams later announced that she was taking a break because of her pregnancy, fans were quick to point out that she was pregnant at the time she lifted the trophy in Melbourne.

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#1. 15-year time span between first and last stint as World No. 1

On July 8, 2002, Serena Williams rose to the coveted World No. 1 ranking for the first time after title triumphs at the French Open and Wimbledon.

Incidentally, she toppled sister Venus, who headed the women's rankings list and stayed at the top of the ranking list for a year before losing the slot to Kim Clijsters of Belgium.

A staggering five-year reign as World No. 1 then followed for Serena, who was perched in the top rung for a record 319 weeks overall. She was World No. 1 before taking a break from the game in 2017 which again was a record, totaling 15 years between reaching the World No. 1 spot for the first time and the last.