When Elena Rybakina takes on Serena Williams for a place in the French Open quarter-finals on Sunday, she will be representing Kazakhstan but there will be envious glances cast from her native Russia.

Rybakina was born in Moscow but switched allegiance to neighbouring Kazakhstan in 2018 when she was struggling at 175 in the world.

Three years on, the 21-year-old is her adopted country's number one with a world ranking of 22.

Ironically, had she stayed loyal she would also now be Russia's highest-ranked player, an honour currently claimed by number 30 Veronika Kudermetova.

Rybakina is the latest in a long line of players to have gone Kazakh in recent years.

"The Kazakhstan Federation made me an offer and the decision was easier," she told WTA Insider in 2020.

"I changed my citizenship to Kazakhstan because they believed in me and they offered. I was not so good when they offered. So they believed in me and they're helping me a lot."

Under the Kazakh flag, Rybakina has won two career titles and taken her personal earnings to over $1.5 million.

Few of the players reveal what kind of sweeteners were offered to entice them to cross the tennis border.

However, the long-standing president of the Kazakhstan Tennis Federation Bulat Utemuratov is estimated by Forbes to be worth $3.5 billion.

Currently, four of Kazakhstan's top five women are Russian-born.

Number two Yulia Putintseva, ranked 43 but a former top 30, is also a Moscow native and is a three-time Grand Slam quarter-finalist.

Rent-a-Russians

In the men's, national number one Alexander Bublik is Russian-born.

The 23-year-old, who is ranked at 37 in the world, switched in 2016. Bublik still lives in Russia.

Kazakhstan's top three is rounded out by Russians -- Mikhail Kukushkin from Volgograd and St. Petersburg native Dmitry Popko.

Kukushkin, now 33 and outside the top 100 having once been in the top 40, was one of the original 'Rent-a-Russians', having switched in 2008.

"At that time I was around 150 in the world and I was struggling," he said.

"I was not in good shape in that moment, but I knew that I could play better, much better and I can get to the other level. 

"But I didn't have any opportunity for that. Unfortunately in Russia nobody was interested in me. Kazakhstan came to me and they provided everything, practice conditions, coaches."

On the international team level, Kazakhstan's most successful singles player in the Billie Jean King Cup is Galina Voskoboeva from Moscow.

In the Davis Cup, Kazakhstan competed in the World Group for the first time in 2011 and have reached the quarter-finals five times.

They will feature in the 2021 Davis Cup Finals in Madrid later this year having beaten the Netherlands in last year's qualifier where Bublik won both his singles rubbers.

On Sunday, Rybakina will shrug off any lingering regrets when she attempts to reach the quarter-finals of a Grand Slam for the first time and halt Williams's push for a record-equalling 24th Slam.

"She is a legend of the sport. I want to be with her on the court, to feel the power and everything," she said.