It was moving day in the California desert, and Roger Federer was up before dawn. We met on the tarmac in Thermal, a short drive from Indian Wells, where Federer had lost the day before in the final of the 2018 BNP Paribas Open to Juan Martín del Potro. Just the previous month, Federer had capped his remarkable late-career surge by reclaiming the No. 1 ranking for the first time in more than five years. At 36, he was the oldest player to hold the spot since the A.T.P. published its first rankings in 1973. But Indian Wells was a rather disappointing sequel. He served for the title against del Potro at 5-4 in the third set and failed to finish him off despite holding three match points.