According to Wiki.
2017: Struggles with form and out of top 20
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Kerber at the 2017 Fed Cup World Group Play-offs
At the start of the 2017 season, Kerber competed in the Brisbane International as the top seed. After receiving a first round bye, Kerber won her opening match against Ashleigh Barty in three sets but then lost to Elina Svitolina in the quarterfinals. Kerber then competed at Apia International Sydney where she received a first-round bye but proceeded to lose her opening match to Russian teenager Daria Kasatkina.
After early lackluster performances, Kerber entered the Australian Open as the defending champion and top seed for the first time at a Grand Slam tournament. She faced Lesia Tsurenko in the first round and defeated her in three sets after squandering match point in the second set. Kerber then defeated compatriot Carina Witthöft in the second round and Kristýna Plíšková in the third round. Kerber then lost to CoCo Vandeweghe in the fourth round winning only five games and ending her title defense. After Serena Williams won the tournament, she amassed enough points to take back her top ranking from Kerber.
Her next two tournaments were the Qatar Open and the Dubai Tennis Championships. At the former, she received a first-round bye before losing to Daria Kasatkina once again. At the latter, Kerber also received a first-round bye and defeated Mona Barthel to set up a rematch of her Olympic final against Monica Puig, which she won in straight sets. She then defeated Ana Konjuh to reach her first semifinal in Dubai, losing there to Svitolina. This defeat prevented Kerber from returning to world No. 1.
At the BNP Paribas Open seeded 2nd, Kerber received a bye into the second round defeating fellow countrywoman Andrea Petkovic, followed by a tight three set win over Pauline Parmentier before losing to eventual champion Elena Vesnina in the fourth round. She lost to Venus Williams in the quarterfinals at the Miami event. Kerber regained the world No. 1 ranking after Serena Williams withdrew before the tournament with a knee injury. In Monterrey she reached the final.
Kerber began the clay court season with a loss in Stuttgart to eventual runner-up Kristina Mladenovic which dropped her from the top slot. Then she retired in the third match of Madrid, but due to Serena's absence became world No. 1 again. At the French Open, Kerber became the first WTA No. 1 in history to be ousted in the first round of the French Open, falling in straight sets to Ekaterina Makarova.
At the start of Kerber's grass court season, she reached the quarterfinals of the Eastbourne International, before being defeated by home favourite Johanna Konta. Kerber went out in the fourth round of Wimbledon, losing to eventual champion Garbiñe Muguruza. Due to Kerber's early exit and Simona Halep's loss to Johanna Konta in the semifinals, Karolína Plíšková became the new No. 1 after the tournament.
Her woes continued into the US hard court swing where she lost to Sloane Stephens in the opening round of the Rogers Cup. She suffered another opening-round loss to Ekaterina Makarova at the 2017 Western & Southern Open despite having a match point, and another first round loss to Naomi Osaka as the defending champion at the US Open. With this loss Kerber dropped out of the top 10.
Her struggles continued beyond the US Open. At the Toray Pan Pacific Open she took revenge on Naomi Osaka and defeated Daria Kasatkina and Karolina Pliskova before falling to Anastasia Pavlyuchenkova. At the Wuhan Open she lost in the first round to eventual champion Caroline Garcia. At the China Open she defeated Naomi Osaka again before losing to Alizé Cornet. At Luxembourg Kerber lost to Monica Puig. Having failed to qualify for the 2017 WTA Finals & losing both her round robin matches at 2017 WTA Elite Trophy, Kerber dropped out of top 20 for the first time since 2012.
On 16 November 2017, Kerber announced on Twitter that she had ended her partnership with longtime coach Torben Beltz and had hired Wim Fissette.[38] Fissette would work with Kerber until 16 October of the following year."
So it's not that uncommon that Kerber can be woeful.
She bounced back in 2018, but doubtful she will in 2020.
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