Roger Federer lost to Ernests Gulbis of Latvia 2-6, 6-1, 7-5 in his opening match at the Rome Masters on Tuesday.
It was Federer's first singles match in nearly a month and his first of the clay-court season. The world's top-ranked player took off several weeks after losing in the fourth round of the Sony Ericsson Open in Key Biscayne on March 30.
The 40th-ranked Gulbis fought off nerves at the end and finally closed out the match on his seventh match point.
Roger Federer of Switzerland hits a return to Ernests Gulbis of Latvia during their match at the Rome Masters tennis tournament April 27, 2010
Federer teamed with Yves Allegro to win a doubles match on Monday at the Foro Italico, but the 16-time Grand Slam champion struggled with his consistency against Gulbis under a light rain.
Federer reached the semifinal here last year and went on to win his first Grand Slam title on clay at the French Open a few weeks later. This year's tournament at Roland Garros begins on May 23.
Federer will play two more tournaments in Estoril and Madrid before heading to Paris.
Federer didn't face a break point until the fourth game of the second set, when he missed a forehand wide to end a long rally and hand Gulbis a 3-1 lead. Gulbis then broke again to go up 5-1 when Federer made a series of errors including a rare overhead miss.
Federer committed 15 unforced errors to Gulbis' eight in the second set alone, and his form in the final set was even worse, missing one routine baseline shot after another.
In all, Federer committed 38 unforced errors to Gulbis' 39, and trailed 20-33 in winners. Federer also struggled with his serve, landing only half of his first attempts to Gulbis' 71 percent.
Earlier, Novak Djokovic began his bid to reach a third consecutive Rome final with a quick 6-1, 6-1 win over French opponent Jeremy Chardy.
Also, Lleyton Hewitt ground out a 6-4, 4-6, 6-3 win over ninth- seeded Mikhail Youzhny, a positive step in the Australian's comeback from hip surgery.
Victor Hanescu of Romania beat another seeded player, No. 16 Juan Monaco, 7-6 (4), 6-4.
In other matches, Stanislas Wawrinka of Switzerland easily beat Juergen Melzer of Austria 6-1, 6-2. Feliciano Lopez of Spain advanced past Benjamin Becker of Germany 6-4, 6-3. Italian wild card Simone Bolelli eliminated German lucky loser Simon Greul 6-4, 6-1