The reasons for their absence are more varied than a 28-shot Monica Niculescu rally, but 2017 saw the game's biggest stars either notably absent for long stretches of the season or mired in tough slumps. The result? The tour's marquee names with some head-scratching rankings - at least to those uninitiated in the tour's revolving 52-week ranking system - which could lead to a series of blockbuster early-round matches in the first few months of the 2018 season.
Angelique Kerber (Current rank: No.21)
Following up her astounding 2016 season was always going to be tough task for the German, but her year-long struggle to find the thunderous brand of counter-punching that won her two majors was one of the year's biggest mysteries. Consistency was elusive from one of the game's most consistent, and the lack of confidence showed in her shot-making.
During the off-season, Kerber parted ways with long-time coach Torben Beltz and hired the ever-experienced Wim Fissette, former coach to Johanna Konta, Simona Halep, and Kim Clijsters. Perhaps a new voice and more temporal distance from the highs of her 2016 season will reveal a new Angie in January.
Serena Williams (Current rank: No.22)
All eyes are on the defending Australian Open champion, as she returns to the tour after giving birth to her daughter in early September. The last time we saw the American star, she broke Stefanie Graf's Open Era record by winning her 23rd major title at the Australian Open and reclaiming the No.1 ranking.
The biggest question surrounding Serena now is both when she'll make her tour-level return - she is expected to face Jelena Ostapenko in an exhibition match on December 30th - and how heavy of a tournament schedule she's expected to play. Serena is just one major title shy of tying Margaret Court for the all-time record in Slam singles titles, and there's no doubt she's gunning to break the record outright.
Agnieszka Radwanska (Current rank: No.28)
The Pole is just 28-years-old, but few players talk about the physical toll of the game on their bodies as much as Radwanska. After six conscutive years finishing inside the Top 10, Radwanska's body finally caught up with her in 2017, where she made just two semifinals (Sydney, New Haven).
Hampered by a foot injury through much of the year, Radwanska was more than relieved when her season came to an end in Hong Kong, finishing the year outside the Top 20 for the first time in over a decade. But that fact alone should indicate her 2017 slump was more an anomaly than a trend. So long as she's healthy, look for the six-time WTA Fan Favorite to bounce back strong in 2017.
Petra Kvitova (Current rank: No.29)
Kvitova's return to the tour after an horrific knife attack in her home in December of 2016 was the most emotional story of 2016. That she returned and was still able to play her audacious brand of tennis only amplified the astounding nature of what she was able to do last season. Upon her reurn to the tour in May, Kvitova won the Aegon Classic, made the US Open quarterfinals, and finished strong with a semifinal run at the China Open. There were erratic dips along the way, but all signs point to the return of Kvitova the contender.
Maria Sharapova (Current rank: No.59)
After serving out her 15-month suspension for an anti-doping violation, Sharapova finished out her up-and-down comeback on a strong note. Derailed by injury after Rome, which ruled her out of the French Open and Wimbledon, she showed she still had what it took to beat the best, beating Simona Halep en route to the Round of 16 at the US Open, and then won her first title since 2015 at the Tianjin Open.
Until Sharapova gets her ranking inside the Top 20, any tournament at which she plays will have a drama-laced draw ceremony, as she'll be the unseeded and looming threat until she's regularly earning a seeding. Get ready for some blockbuster early round matches.
Victoria Azarenka (Current rank: No.210)
The two-time Australian Open champion's comeback after giving birth to her son Leo lasted just two tournaments last season, one of which was a strong run to the Round of 16 at Wimbledon. Since then, the 28-year-old has paused her return for personal reasons, but here's hoping we see her back on the baseline brawling soon in 2018.