PARIS (AP) — Through 14 years and 21 matches, Serena Williams vs. Maria Sharapova has evolved into one of the most enduring and multi-faceted, if also lopsided, rivalries in women's sport.

The next episode comes Monday, in their fourth-round match at the French Open.

Beating Sharapova, at the Australian Open semifinals in 2005, the start of Williams' current 18-0 streak against the Russian. "Whenever I play her, obviously I go out there the way I play everyone else. I go extremely hard," Williams said ahead of their latest encounter at the French Open.

They speak with respect for each other's tennis and achievements. They are among the highest-paid women athletes in history, having banked more than $100 million in prize money between them. They both had tough childhoods. And with their intense, almost primeval competitive drive, they have blazed trails in women's tennis and forced the next generation of players to up their game.

Yet for all their similarities, there is little love lost between them, with spats punctuating their sometimes fraught relationship.

Here's a look at the rivals:

VIDEO: Williams takes a swipe at Sharapova's biography: "The book was 100 percent hearsay."

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Their last meeting at Roland Garros, the 2013 final won 6-4, 6-4 by Williams.

ON COURT: If not for Williams, Sharapova would likely have more than five major titles by now. Sharapova has lost seven of the eight times they've met in Grand Slams, including two finals at the Australian Open and the 2013 French Open final.

Overall, Williams has a 19-2 record against the Russian.

Sharapova's wins against Williams both date back to 2004. Aged just 17, she stunned the-then two-time defending champion by taking her Wimbledon crown and beat Williams again four months later in the final of the WTA Championships.

The WTA Tour Championships in Nov. 2004. "For a Serena year, it wasn't superb," Williams said. "An extraordinary year for me," Sharapova said.

Since then, it's been 18-0 for Williams.

Sharapova has taken just three sets off Williams in that time and none in their last seven matches since 2013.

Tennis superstars for more than a decade, Williams and Sharapova have banked tens of millions of dollars and blazed trails since this photo in 2004. But Williams has enjoyed the bulk of the on-court success. "There is a lot of things in her game that she's done much better than I have," Sharapova says. "Numbers don't lie."

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OFF COURT: Both formerly ranked No. 1, they're coming back from very different off-court challenges.

Sharapova tested positive for meldonium in 2016, weeks after the World Anti-Doping Agency banned the drug used by many Russian and Eastern European athletes. She served a 15-month ban.

The Australian Open final, 2015. A 6-3, 7-6 (5) victory for Williams. A year later, at the same tournament, Sharapova tested positive.

Williams had a difficult recovery from an emergency cesarean section and post-partum complications after the birth of her daughter, Alexis Olympia, last September.

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FORMATIVE YEARS: Sharapova's parents, Yelena and Yuri, fled their city of Gomel in Belarus because of the nuclear reactor disaster in Chernobyl. Yelena was pregnant with her only child at the time. At 6, Sharapova and her father moved to Florida, separating them from her mother because of visa restrictions and limited finances.

Yuri embraces his daughter after she beats Williams at Wimbledon. In her biography published last year, Sharapova says of Williams: "I approach every match against her with trepidation and respect."

Serena and her sister Venus learned tennis on crumbling courts in Compton, California, once one of the most dangerous U.S. cities because of its high murder rate. Their half-sister, Yetunde Price, was killed in a gang shooting.

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SPATS: Williams warmed up for their match Monday by taking a swipe at Sharapova's biography, "Unstoppable: My Life So Far," published last year. Williams wasn't happy that it talks about her crying in the Wimbledon locker room in 2004.

They also had an off-court tiff in 2013, taking digs at each other's love lives.

Wimbledon, 2004. Fourteen years later, at this French Open, this final again became a talking point. Williams complained about Sharapova describing her post-match locker room tears in her biography. "What happens there should definitely maybe stay there and not necessarily talk about it in a not-so-positive way in a book," Williams said.