London - Stan Wawrinka's Wimbledon preparations suffered a blow as the Swiss star crashed to a 7-5, 6-7 (3/7), 6-1 defeat against Sam Querrey at Queen's Club on Wednesday.
Wawrinka has struggled to return to peak form since sustaining a left knee injury last year that required two operations and wiped out the second half of the Swiss star's season.
Still hampered by the problem this term, Wawrinka was sidelined again for three months in between a second round exit at the Australian Open and a first round loss in the French Open.
The three-time Grand Slam champion had swatted aside British youngster Cameron Norrie in the first round of the Wimbledon warm-up at Queen's.
But Wawrinka faded badly in the final set against Querrey as the American world number 13 condemned the 33-year-old to his seventh defeat in his last nine ATP Tour matches.
It is a major setback for Wawrinka ahead of the July 2 start of Wimbledon, where he has never made it past the quarter-finals.
Querrey, who won Queen's in 2010, will play Croatian top seed Marin Cilic in the last eight.
Cilic maintained his mastery over Luxembourg's Gilles Muller, hitting back to win 4-6, 6-3, 6-3 in their latest big-serving battle.
Chasing his second Queen's title and his fourth final appearance, Cilic once again frustrated Muller 12 months after beating him in the final set in both the Wimbledon quarter-finals and Queen's semi-finals.
Cilic, who lost last year's Queen's final against Feliciano Lopez, hit 19 aces to 17 from Muller as he made the quarter-finals for the seventh time.
American prodigy Frances Tiafoe moved into his first ATP Tour grass-court quarter-final with a 6-4, 2-6, 6-4 win against Argentina's Leonardo Mayer.
Tiafoe, ranked 62nd, is looking to add a second ATP title to his maiden success at Delray Beach in February.
The 20-year-old out-lasted Mayer to stop the world number 38 building on his surprise first round victory against third seed Kevin Anderson.
Tiafoe next faces France's Jeremy Chardy, who beat Russia's Daniil Medvedev 7-6 (8/6), 6-3.
Results at the ATP Queen's tournament in London on Wednesday (x indicates seed):
Marin Cilic (CRO x1) bt Gilles Muller (LUX) 4-6, 6-3, 6-3
Sam Querrey (USA x5) bt Stan Wawrinka (SUI) 7-5, 6-7 (3/7), 6-1
Jeremy Chardy (FRA) bt Daniil Medvedev (RUS) 7-6 (8/6), 6-3
Frances Tiafoe (USA) bt Leonardo Mayer (ARG) 6-4, 2-6, 6-4