Wawrinka beaten on return from injury in Rome

"I really only started to play tennis again 12 days ago," said Switzerland's Stan Wawrinka, a three-time Grand Slam champion. (AFP/ANNE-CHRISTINE POUJOULAT)

The three-time Grand Slam champion lost 6-4, 6-4 to 55th-ranked American Steve Johnson.

"I really only started to play tennis again 12 days ago," said Wawrinka, who was broken in each set and hit 30 unforced errors to Johnson's 17.

"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."

Wawrinka, 33, sat out the second half of 2017 after a knee operation following a first-round exit at Wimbledon in June.

He then spent eight weeks on crutches following surgery to repair the damage.

He returned this year but has struggled and was forced to withdraw during a second round match in Marseille in February against world number 193 Ilya Ivashka of Belarus.

"I think when I came back in Australia it was quite clear I wasn't ready at all. But for me it's important to play matches to test myself playing against the top guys," added Wawrinka with the French Open, which he won in 2015, just two weeks away.

"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."

Three Americans advanced on the first day on the red clay in Rome with 13th seed Jack Sock beating Spaniard David Ferrer 6-3, 6-4, and Ryan Harrison ousting 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 defeat France's Adrian Mannarino 2-6, 7-6 (7/4), 6-3.

Seven-time Rome winner Rafael Nadal is the top men's seed as the world number one competes after suffering his first clay-court defeat in almost a year in the Madrid quarter-finals to Austrian Dominic Thiem.

The women's tournament at the Foro Italico starts on Monday, with two-time Wimbledon champion Petra Kvitova withdrawing citing a muscular injury after winning Saturday's Madrid Open final, her second title in two weeks after also taking the Prague Open.

Four-time former Rome champion Serena Williams withdrew last week as she is struggling to be fit after having a baby last year.

"After these incredible two weeks of tennis my body needs to rest and recuperate," said 28-year-old Czech Kvitova.

World number one Simona Halep is the top women's seed.

Men’s first round results:

Lorenzo Sonego (ITA) bt Adrian Mannarino (FRA) 2-6, 7-6 (7/4), 6-3

Steve Johnson (USA) bt Stan Wawrinka (SUI) 6-4, 6-4

Jack Sock (USA) bt David Ferrer (ESP) 6-3, 6-4

Ryan Harrison (USA) bt Yuichi Sugita (JPN) 7-6 (7/5), 6-3

Peter Gojowczyk (GER) bt Sam Querrey (USA) 6-2, 7-6 (9/7)