She’s seeded fourth and is bringing her staggering serve to the event in Cancun, Mexico.Elena Rybakina was mad. She expressed it in the manner most comfortable to her.“Thank you for changing the rules last moment,” Rybakina wrote in an Instagram story last month, alongside a clapping emoji. “Great decisions as always @wta.” She ended with a circus tent and clown-face emojis.Rybakina, who will be making her debut in the season-ending WTA Finals in Cancun, Mexico, on Sunday, had arrived in Tokyo for a tournament expecting, as the No. 3 seed, to receive a bye. That meant one less match to play and a few days of extra rest.But, because of a rather technical WTA rule, Maria Sakkari and Caroline Garcia, who had just reached the semifinals at a tournament in Guadalajara, Mexico, and had a long distance to travel before the start in Tokyo, were given what’s known as “performance byes.” That meant that Rybakina no longer had a free pass into the second round.Rybakina celebrating after winning a match against Aryna Sabalenka at the China Open earlier this month. Rybakina beat both Sabalenka and Iga Swiatek, the world’s top two players, this season.Wu Hao/EPA, via ShutterstockSo, after the draw was already made, Rybakina withdrew from the tournament. She said, at the time, it was because of a lingering illness that had plagued her since the summer. (Rybakina declined to be interviewed for this article.)It was not the first time that Rybakina found herself drawn into controversy. After her semifinal win over Victoria Azarenka at the Australian Open in January, she defended her longtime coach, Stefano Vukov, when he was criticized for yelling at her midmatch.“He is a passionate coach with a lot of knowledge about tennis,” Rybakina — who lost the Australian final to Aryna Sabalenka in three sets — wrote in an Instagram post, responding to the commentary. “Those who know me well, will know that I would never accept a coach that didn’t respect me and all our hard work.”Then, during a tournament in Montreal in August, Rybakina went on the offensive after a persistent rain delay forced her to begin her quarterfinal match after 11 p.m. Her match against Daria Kasatkina (which she eventually won) didn’t end until almost 3 a.m. She then lost her ensuing semifinal and complained of niggling injuries.This type of grumbling is unusual for Rybakina, 24, who typically goes about her business on court with little indication of what she is thinking or feeling.“Elena is so professional and very focused, and she really controls her emotions,” said her agent, Nicholas Tzekos. “You have to spend quantity time with her to get to the quality. But when she does smile, it’s as if she’s giving a giant hug to the whole audience.”Rybakina’s game is buoyed by her six-foot frame and a staggering serve that has routinely been clocked in the mid-120 m.p.h. range, as well as a forehand that she can redirect into the furthest corners of the court.Rybakina has won two tournaments this year, at Rome and Indian Wells, winning enough ranking points to earn a spot at the WTA Finals this week.Pedro Pardo/Agence France-Presse — Getty Images“Her serve is amazing,” said Billie Jean King, who will welcome Rybakina to the Billie Jean King Cup Finals in Seville, Spain, days after the WTA Finals. “Her forehand gets a little dodgy sometimes, but she’s always going to be a threat because she loves to play and she believes in herself.”Rybakina had her big breakthrough at Wimbledon last year, defeating Ons Jabeur for the title. The Moscow-born Rybakina had been representing Russia, but chose instead to start playing for Kazakhstan in 2018; she won Wimbledon while competing for the Central Asian nation.That year, the tournament had banned Russian and Belarusian players because of Russia’s invasion of Ukraine. The WTA then opted not to award ranking points for Wimbledon, meaning that Rybakina, despite being one of the best players that season, failed to qualify for the 2022 WTA Finals in Fort Worth.This year, Rybakina, ranked No. 4 and seeded 4th at the Finals, has won two tournaments, the BNP Paribas Open at Indian Wells, where she beat the world’s top two players, Iga Swiatek and Sabalenka, in succession, and the Internazionali BNL d’Italia in Rome. With three wins over Swiatek and one over Sabalenka, she is one of just two WTA players since 2000 to have four wins over world No. 1s in the same season.The other was Serena Williams. More