After his 6-3 7-6 (7-4) defeat to Jan-Lennard Struff, a disappointed Tommy Haas hinted it could be his last match on the ATP Tour.
As Donald Young reached the Citi Open second round, Daniil Medvedev saved a match point to battle through in Washington.
Making her return from injury, Maria Sharapova overcame Jennifer Brady in the Bank of the West Classic first round.
Julia Goerges and Oceane Dodin advanced at the Citi Open as third seed Lauren Davis slumped to a first-round defeat.
Andrey Kuznetsov was on the receiving end of a shock defeat at the Generali Open, losing to Miljan Zekic in Kitzbuhel.
Not since July 2007 has Novak Djokovic been out of the top four in the rankings, but the injured Serbian dropped to fifth on Monday.
American John Isner served 23 aces to edge past Ryan Harrison and win the Atlanta Open for the fourth time.
Katerina Siniakova improved on her runner-up finish in the 2016 Swedish Open by taking the 2017 title, beating top seed Caroline Wozniacki.
Leonardo Mayer became the second lucky loser in as many weeks to win an ATP title, beating Florian Mayer at the German Tennis Championships.