Stan Wawrinka's Wimbledon preparations were dealt a blow as he suffered a 7-5, 6-7 (3-7), 6-1 defeat against Sam Querrey at Queen's Club on Wednesday.
The Swiss star has struggled to return to full fitness since suffering a knee injury last year that required two surgeries.
He returned at the Australian Open at the start of the year, but was then forced into another three-month lay-off in February.
Wawrinka would have been hoping for a better showing on grass after a first-round exit at Roland Garros, but he came unstuck against big-hitting Querrey on Wednesday.
While he fought back bravely after dropping the first set to take the second in a tie-break, Wawrinka faded badly in the decider to slump to a seventh defeat in his last nine ATP Tour matches.
In other second-round results, top seed Marin Cilic fought back from a set down to defeat Gilles Muller 4-6, 6-3, 6-3, while Jeremy Chardy beat Daniil Medvedev 7-6 (8-6), 6-3.
Rising star Frances Tiafoe was another winner, moving into his first ATP grass-court quarter-final with a 6-4, 2-6, 6-4 win over Leonardo Mayer.