Dan Evans and Juan Martin del Potro have both withdrawn from next week's Aegon Championships at Queen's Club because of injury.
British number three Evans faces a race to be fit for Wimbledon after suffering a calf injury at a tournament in Surbiton last week.
The 27-year-old had to retire during his quarter-final against Dustin Brown and was seen on crutches after pulling out of this week's Aegon Open in Nottingham.
Former US Open champion Del Potro is still struggling with the groin problem that hampered him at the French Open.
James Ward has been given a wild card into the tournament as he continues his comeback from a knee injury.
The 30-year-old, a key member of Great Britain's Davis Cup winning team in 2015, was a semi-finalist at Queen's in 2011 but is currently ranked outside the top 1,000.