I preview world numbers 4-8 in the ATP year-end rankings. 
- Dominic Thiem improved his shot selection in a career-best 2019, but there's still room to grow there. Do I see a major title in his 2020? 
- Daniil Medvedev was nearly unbeatable in the summer of 2019, but can he keep it up? 
- Stefanos Tsitsipas started and ended last season strong. What can he do to be more consistent throughout 2020. 
- Alexander Zverev has yet to reach his full potential. It comes down to off-court stability and some major tactical needs for improvement. 
- Matteo Berettini burst onto the scene. How far can he ride his big serve and forehand?