expressed gratitude toward fans and called the win an "honor." 2015: WTA Finals champion At the end of the 2014 season, Radwańska hired Martina Navratilova as part of her coaching team, with the specific goal of winning a Grand Slam title. She started 2015 by teaming with Jerzy Janowicz to win the Hopman Cup, Poland's first title in the event. The victory included Radwańska's first win over Serena Williams in a singles match. Radwańska competed in Sydney, where she defeated Alizé Cornet but lost to Garbiñe Muguruza in the second round. She reached the fourth round of Australian Open, where she lost to Venus Williams for the first time since 2010, in three sets. In February, Radwańska played in Dubai and Doha, where she recorded wins over Caroline Garcia and Flavia Pennetta, but lost for the second time in a year to Muguruza and Venus Williams, respectively. She then progressed to Indian Wells where she lost in the third round to Heather Watson. After that, she competed in the Miami Open, where she lost to Carla Suárez Navarro in the fourth round. In April, Radwańska participated for the second time in her career at Katowice Open in Poland, where she defeated Yanina Wickmayer, An-Sophie Mestach, Klára Koukalová before losing to Camila Giorgi in the semifinals. Shortly after Radwańska's first-round loss at Stuttgart Open, Martina Navratilova stated that she was too busy to continue in her part-time role as Radwańska's consulting coach. Radwańska stated, "I just wanted to thank Martina for her time and efforts over these last few months. It was a great experience to work with one of the all-time greats. However, we both agreed that as Martina could not commit 100% to the project then it was not going to work as a long-term partnership." At the Madrid Open, Radwańska defeated Lara Arruabarrena and Casey Dellacqua before losing to Caroline Wozniacki in the third round. As a result of this loss, Radwańska dropped outside of the Top 10 for the first time since October 2011. At the French Open, Radwańska was defeated by Annika Beck in the first round. The loss marked only her third first-round loss at a Grand Slam in her career, and her first since the 2009 Australian Open. In June, Radwańska competed in her first grass-court tournament at Nottingham where she advanced to the semifinals, losing to Monica Niculescu. In her next grass tournament