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.
Last month Murray committed to the championships for the rest of his career and the BBC announced it will cover the event until at least 2024.
His brother Jamie won the US Open and Australian Open doubles with Brazilian Bruno Soares last year, although his partner at Queen's will not be confirmed until nearer the time.
This year's tournament will run from 19-25 June.