ROME -- Stan Wawrinka is still struggling to regain his form following surgery on his left knee.
Playing his first match since February, the three-time Grand Slam champion lost to 55th-ranked Steve Johnson 6-4, 6-4 Sunday in the first round of the Italian Open.
Wawrinka committed 30 unforced errors to his American opponent's 17, was broken once in each set and failed to take advantage of his only two break opportunities.
Wawrinka was operated on in August -- twice -- and returned in January at the Australian Open, where he was eliminated in the second round by Tennys Sandgren. He then retired from a match against 193rd-ranked Ilya Ivashka in Marseille, France, in February, citing a left knee injury.
Three other Americans were also in action on the Foro Italico's red clay.
Twelfth-seeded Sam Querrey was eliminated by Peter Gojowczyk of Germany 6-2, 7-6 (7); 13th-seeded Jack Sock beat Spanish veteran David Ferrer 6-3, 6-4; and Ryan Harrison eliminated Yuichi Sugita of Japan 7-6 (5), 6-3.
Also, Italian wild card Lorenzo Sonego had strong fan support as he came back from a set down to defeat Adrian Mannarino 2-6, 7-6 (4), 6-3.
Meanwhile, two-time Wimbledon champion Petra Kvitova withdrew from the tournament citing a muscular injury that occurred during a grueling three-set win over Kiki Bertens in Saturday's Madrid Open final -- her second title in two weeks after also taking the Prague Open.
"After these incredible two weeks of tennis my body needs to rest and recuperate," Kvitova said.
The women's tournament at the Foro Italico starts Monday.
It's the last Masters 100 event before the May 27-June 10 French Open