The key for Roger's winning these days is not his serve, contrary to popular belief.
It is the quality of his return play.
In all of his losing matches of the past two years, his returns points won percentage fell below 34%.
4‑Mar‑2019 Indian Wells Masters (7)Dominic Thiem [AUT] d. (4)Federer 3-6 6-3 7-5 33.0%
14‑Jan‑2019 Australian Open (14)Stefanos Tsitsipas [GRE] d. (3)Federer 6-7(11) 7-6(3) 7-5 7-6(5) 27.7%
12‑Nov‑2018 Tour Finals (3)Alexander Zverev [GER] d. (2)Federer 7-5 7-6(5) 20.0%
12‑Nov‑2018 Tour Finals (7)Kei Nishikori [JPN] d. (2)Federer 7-6(4) 6-3 29.4%
29‑Oct‑2018 Paris Masters (2)Novak Djokovic [SRB] d. (3)Federer 7-6(6) 5-7 7-6(3) 25.7%
8‑Oct‑2018 Shanghai Masters (13)Borna Coric [CRO] d. (1)Federer 6-4 6-4 18.4%
27‑Aug‑2018 Us Open John Millman [AUS] d. (2)Federer 3-6 7-5 7-6(7) 7-6(3) 30.6%
13‑Aug‑2018 Cincinnati Masters (10)Novak Djokovic [SRB] d. (2)Federer 6-4 6-4 26.4%
2‑Jul‑2018 Wimbledon (8)Kevin Anderson [RSA] d. (1)Federer 2-6 6-7(5) 7-5 6-4 13-11 31.7%
18‑Jun‑2018 Halle Borna Coric [CRO] d. (1)Federer 7-6(6) 3-6 6-2 32.7%
19‑Mar‑2018 Miami Masters (Q)Thanasi Kokkinakis [AUS] d. (1)Federer 3-6 6-3 7-6(4) 33.6%
5‑Mar‑2018 Indian Wells Masters (6)Juan Martin Del Potro [ARG] d. (1)Federer 6-4 6-7(8) 7-6(2) 27.8%
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Note that there were only 3 matches in 2018-2019 that Federer won when his return point won pct. went below 33%:
25‑Feb‑2019 Dubai (2)Federer d. Fernando Verdasco [ESP] 6-3 3-6 6-3 30.0%
18‑Jun‑2018 Halle (1)Federer d. (Q)Denis Kudla [USA] 7-6(1) 7-5 32.4%
11‑Jun‑2018 Stuttgart (1)Federer d. (7)Milos Raonic [CAN] 6-4 7-6(3) 27.9%
The key to getting these wins was that his opponents' return percentage was even worse than Federer's. Verdasco: 25.6%, Kudla: 26.7%, Raonic: 22.7% - Of course, return win pct. is generally expected to be lower on grass or on faster surfaces.
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In Miami, here were Federer's return point wonpercentages:
(4)Federer d. (7)John Isner [USA] 6-1 6-4 43.4%
(4)Federer d. (20)Denis Shapovalov [CAN] 6-2 6-4 44.8%
(4)Federer d. (6)Kevin Anderson [RSA] 6-0 6-4 52.0%
(4)Federer d. (13)Daniil Medvedev [RUS] 6-4 6-2 39.2%
(4)Federer d. Filip Krajinovic [SRB] 7-5 6-3 43.9%
(4)Federer d. (Q)Radu Albot [MDA] 4-6 7-5 6-3 36.6%
Federer's return was very sharp for the last 3 matches and it needed to be, and was significant in earning him the wins.
Respectfully,
masterclass