Cilic reached the third round of Wimbledon this year
Sixth seed Marin Cilic suffered a shock second-round defeat by Indian qualifier Somdev Devvarman in the ATP Legg Mason Tennis Classic in Washington DC.
The Croat produced numerous unforced errors during the 7-5 6-4 loss to world number 153 Devvarman.
Defending champion Juan Martin del Potro came from a set down to beat Taiwan's Yen-sun Lu 4-6 6-3 6-2.
Eighth-seeded Czech Tomas Berdych downed Lativa's Ernests Gulbis 6-2 7-6 (8-6) to book his third-round spot.
World number 15 Cilic showed signs of rustiness in his encounter with Devvarman having received a bye in the first round, but was still expected to prevail over an opponent he defeated in the final at Chennai, India in January.
However, Devvarman was able to capitalise on Cilic's lack of match practice to record his first win over a top-20 ranked player.
"I knew that it was the first match he was playing this week and I already had three under my belt. That's always a good feeling to bring to the match," said Devvarman.
Roddick sets sights on US Open
He will now face either Rainer Schuettler of Germany, who beat Argentine Leonardo Mayer 6-1 3-6 7-6 (7-5), or Croat Ivo Karlovic in round three.
In a match that started late on Tuesday night and ended well past midnight, Del Potro lost the opening set after dropping his serve at 3-3 but stormed back with his own break at 4-3 to give him the winning edge in the second set.
After breaking Lu to open the third set, the 2008 champion finished off the long night by winning 16 of 18 service points.
Del Potro now faces a third-round tie with either Australian Lleyton Hewitt or Israeli number 15 seed Dudi Sela.
German Tommy Haas, the only other seeded player in action on Tuesday, edged past Canadian Frank Dancevic 6-1 4-6 6-4 to set up a third-round match with either Tommy Robredo or Juan Carlos Ferrero.
Spaniard Ferrero beat Nicolas Lapentti of Ecuador 6-4 6-7 (5-7) 6-0 to advance to round two while Robredo was granted a first-round bye.
Germany's Benjamin Becker beat American Robby Ginepri 7-6 (7-3) 6-7 (1-7) 7-6 (7-4) to set-up a second-round meeting with top seed Andy Roddick, who has not played since injuring his right hip in his epic loss to Roger Federer in the Wimbledon final a month ago.
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