Stan Wawrinka insists he is "close to his top level" despite struggling on his return after almost three months on the sidelines at the Rome Masters on Sunday.
The Swiss star suffered a 6-4 6-4 defeat to 55th-ranked American Steve Johnson in the first round of the clay-court tournament in the Italian capital.
Wawrinka sat out the second half of 2017 after a knee operation and then spent eight weeks on crutches following surgery to repair the damage. He returned to action this year but has struggled and was forced to withdraw during a second round match against Ilya Ivashka of Belarus in Marseille in February.
But the 33-year-old three-time Grand Slam champion says the early signs are positive despite the French Open, which he won in 2015, just two weeks away.
"I really only started to play tennis again 12 days ago," said Wawrinka, who was broken in each set and hit 30 unforced errors. "I'm happy with physically where I am right now. I had a lot of hesitation with my game and it obviously makes a big difference in the way I move and the way I play."
"It's positive to see that the knee was doing okay, but now I need a lot of time to work on my fitness. Today was tough but I'm really happy to see where I am right now.
"I'm close to my top level, I still need a lot of time to play matches."
In the day's other matches, 13th seed Jack Sock beat Spaniard David Ferrer 6-3 6-4, and Ryan Harrison ousted Japan's Yuichi Sugita 7-6 (7-5), 6-3. But 12th-seeded American Sam Querrey fell 6-2 7-6 (9-7) to Germany's Peter Gojowczyk while Italian wild card Lorenzo Sonego came back from a set down to upset France's Adrian Mannarino 2-6 7-6 (7-4) 6-3.
On Monday, former world No 1 Novak Djokovic takes on Alexandr Dolgopolov while home favourite Fabio Fognini takes on flamboyant Frenchman Gael Monfils in what could be one to watch and British No 1 Kyle Edmund tackles Malek Jaziri from Tunisia.
In the women's draw, Johanna Konta faces 17th-seeded Magdalena Rybarikova from Slovakia in the first round.
The Masters 1000 Series continues with our coverage of the ATP Italian Open on Sky Sports Arena starting from 10am on Monday.
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