It is almost the end of an era.

It is difficult to imagine tennis without the American legend, who has won all there is to win during her 27-year career.

From a purely sporting point of view, Williams will be remembered as one of the game's greatest competitors; an athlete who possessed the purest serve in the women's game and a never-fading desire to win.

She will also be remembered for speaking out against racism, pushing for gender parity and equal prize money, as well as her experiences of the healthcare system as a black woman.

Here, BBC Sport looks at the big moments from an iconic career.

First Grand Slam win - US Open 1999

Aged just 17, Williams produced a fine run to lift her first Grand Slam singles title in New York. She beat world number four Monica Seles in the quarter-finals, then second-ranked Lindsey Davenport before meeting world number one Martina Hingis in the final.

Williams claimed a straight-set victory to become the first African-American woman in the Open Era to win a singles major. Just for good measure, she and Venus then won the doubles title together in the same weekend.

Addressing racism - Indian Wells 2001

Both Williams sisters have spoken about the racism they experienced throughout their career in a traditionally white sport and the issues their father, Richard, faced.

The sisters were set to meet in the semi-finals of Indian Wells - one of the biggest events outside of the Grand Slams - in 2001. However, when Venus withdrew with injury, accusations of match-fixing were pointed towards Richard and the sisters. Serena was then heavily booed when she came out to play Kim Clijsters in the final, and both Richard and Venus said they were racially abused by the crowd.

Serena won and instantly hugged her father in the stands. She and Venus then boycotted the event for many years, with Serena not returning until 2015 and Venus a year later.

Completing the 'Serena Slam' - Australian Open 2003

Williams arrived in Melbourne on the back of an amazing Grand Slam streak, having won every major since the 2002 French Open. Only the Australian Open trophy was absent from her cabinet.

She had to do it the hard way in Melbourne, saving two match points and overturning a 5-1 third set deficit against Kim Clijsters in the semi-finals to reach the final. She then beat Venus for her fourth consecutive major singles triumph. It also secured a career Grand Slam and a non-calendar year Grand Slam for Williams at just 22 years old.

Double triumph at SW19 - Wimbledon 2012 & London Olympics

Williams' triumphs on the Wimbledon grass were the start of a resurgent few years that saw her win another nine major singles titles.

She followed up her Wimbledon title run with a dominant performance to claim Olympic singles gold. Williams thrashed Maria Sharapova 6-0 6-1 in just 62 minutes on the same court where Sharapova had so memorably beaten her in 2004. It was the heaviest defeat for Sharapova against Williams, with the American winning 22 of their 24 meetings over 15 years.

Another 'Serena Slam' before US Open heartbreak - 2015

Williams started and ended the 2015 season as world number one, winning the Australian Open, Roland Garros and Wimbledon to go with her 2014 US Open title and secure yet another 'Serena Slam'.

She was on course for the calendar Grand Slam in New York and faced Italy's Roberta Vinci in the semi-finals, who she had beat four times in a row. She fought back from dropping the first set and led 2-0 in the decider before the unseeded Italian came back to stun Williams and the crowd. Williams later described the loss as one that broke her heart.

Winning number 23 at eight weeks pregnant - 2017 Australian Open

Williams found out she was expecting her first child shortly before the first Grand Slam of the year. At eight weeks pregnant she clinched her eighth Australian Open title and passed Steffi Graf in the all-time list of major winners in the Open Era, with only Margaret Court ahead of her.

She did not drop a set in Melbourne and, to underline her dominance, returned to the top of the world rankings before going on maternity leave.

The 'superhero catsuit' - French Open 2018

Williams has spoken extensively about the struggles black women in the United States experience in healthcare, particularly in regards to complications in pregnancy or childbirth. Williams herself almost died after giving birth to Olympia because of an embolism.

She made her Grand Slam return at Roland Garros eight months after giving birth and wore a black catsuit which, as well as making her feel like 'a queen from Wakanda', helped her cope with blood clots. Two months later, she would reach the Wimbledon final, where she lost to Angelique Kerber.

The controversy in New York - US Open 2018

Williams said several times that breaking Court's record was "the only reason" she still played following her daughter's birth and it seemed fitting that she could equal it on her home turf after reaching the US Open final.

Williams was the heavy favourite against maiden finalist Naomi Osaka. However, Williams' outburst at umpire Carlos Ramos, where she called him a liar and thief after being docked three points, divided opinion and led to a hostile New York crowd. Osaka triumphed but both players were in tears by the end.

The long-awaited return - Wimbledon 2022

With rumours swirling of retirement, it felt cruel that the last time Wimbledon would see the American great would be when she limped off in tears in the first round in 2021 after injuring her ankle.

However, her surprise decision to compete in singles through a wildcard led to a late-night thriller against France's Harmony Tan. Williams thrilled the crowd and showed all her competitiveness before going down in three sets.

As she left to a standing ovation, she stood and twirled one last time on the stage that has brought her and the fans so much joy over the years.

There will, quite simply, never be another Serena.