US Open champion Marin Cilic and world number seven Stan Wawrinka are the latest players to agree to play in the Aegon Championships at Queen's Club.
The pair join British number one Andy Murray, defending champion Grigor Dimitrov and four-time champion Lleyton Hewitt in the tournament line-up.
"The atmosphere is always something special. I have some very happy memories," said 2012 champion Cilic.
The championships, broadcast on the BBC, will be held from 15 June to 21.
Wawrinka, the 2014 Australian Open champion, reached the semi-finals last year.
"It was the furthest I had gone on grass and after that I went on to have my best result at Wimbledon by reaching the quarter-finals, so it was obvious to me that this was the right way to prepare," said the Swiss, 29.
World number five Murray, who lost in the third round last year, will target a record-equalling fourth title, which would emulate John McEnroe, Boris Becker, Hewitt and Andy Roddick.