US Open champion Stan Wawrinka and in-form Grigor Dimitrov will be in the line-up for the Aegon Championships at Queen's Club in London in June.
Britain's Jamie Murray has committed to compete in the doubles.
Bulgarian Dimitrov, 25, won the Queen's tournament in 2014 - and believes he has improved since then.
"I feel like I'm a better player, a better person, better everything," said Dimitrov, who has won titles in Sofia and Brisbane this year.
Dimitrov and Swiss Wawrinka join Scottish world number one Andy Murray and 14-time Grand Slam champion Rafael Nadal in the singles field at Queen's.