His victory against Nishikori at the Canadian Masters marked the first time in seven years that Monfils had come from a set down to defeat a Top 10 opponent and it was just the third time in his career (3–62) that he had done so. Monfils saw his year-end position in the ATP singles rankings slip to world No. 46 as he struggled with his fitness in 2017. He was forced to withdraw from six tournaments because of various ailments and concluded his season after losing in the third round of the 2017 US Open, citing a right knee injury. Monfils won his seventh ATP World Tour singles title at the 2018 Qatar Open. He had lost his previous three Qatar Open singles finals. Appearing in the 2018 edition of the tournament as an unseeded wildcard, he advanced to final via walkover after top seed Dominic Thiem had to withdraw from their match in the semifinals because of illness. In the final, he defeated Andrey Rublev 6–2, 6–3 in 61 minutes. In his next tournament, the 2018 Australian Open, Monfils lost to the No. 14 seed Novak Djokovic in four sets in the second round. Monfils then made the semifinals of the 2018 Argentina Open, upsetting seventh seed Pablo Cuevas in the first round before being handily defeated by Dominic Thiem in the semifinals. Continuing his South American swing, Monfils played two stunning matches during the 2018 Rio Open. In round one, Monfils completed a significant comeback to beat Horacio Zeballos, saving one match point in the process with a booming forehand. In round two, the Frenchman pulled off an even bigger stunner, defeating top seed and world No. 3 Marin Čilić in a match that stretched over two days due to inclement weather. The match was halted at 6–3, 6–6(7–7) in Monfils' favor, a stoppage that surprised many. Monfils lost to Diego Schwartzman, the eventual champion, in the quarterfinals. Later in the year, Monfils finally made the fourth round of Wimbledon for the first time in his career, losing to Kevin Anderson. Monfils won the eighth ATP Tour singles title of his career at the 2019 Rotterdam Open, defeating Stan Wawrinka in a three-set final. At the Madrid Open, the fifthteenth seeded Monfils failed to convert two match points when he was leading 6–5 in the third and final set of his third round match against the No. 4 seed Roger Federer; Monfils eventually lost the match 0–6, 6–4, 6–7(3–7).