London, Jun 20 (AFP) 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 today.

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 or Luxembourg's Gilles Muller in the quarter-finals.

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. (AFP) ATK