Croatian fifth seed Marin Cilic began his title defence at the Queen's club in impressive style by defeating Chilean world No 32 Cristian Garin 6-1, 7-6 on Monday.
Cilic, who beat current world No 1 Novak Djokovic in last year's final, continued as he left off from 12 months ago, blasting Garin away in an opening set that last just over 20 minutes.
The 2014 US Open champion looked to have set himself up for an equally comfortable second set after breaking Garin in the first game. However, a dip in concentration in the fourth game saw Cilic get broken back, although he immediately re-claimed the advantage to lead 3-2.
Yet, when Cilic was in position to serve for the match, he slipped up again as Garin levelled for 5-5 as the second set went to a tie-break. The Chilean then quickly raced into a 4-0 lead with the match seemingly heading towards a decider.
However, Cilic roared back to win the next four points before claiming the match and his place in the second round and a showdown with Argentine Diego Schwartzman, who was a comfortable winner over Kazakhstan's Alexander Bublik.
Wimbledon finalist Kevin Anderson is also through to the second round after battling back from a set down to begin his grass court season with a 4-6, 7-6, 6-4 victory over Britain's Cameron Norrie.
Anderson returned to court for the first time since March after missing the clay court season due to a lingering elbow injury and looked short on match practice as Norrie claimed the first set 6-4.
The world No 8 slowly began to find his groove in the second, but still needed a tie-break to take the match into a deciding set.
Norrie's serve was finally broken to give Anderson a 3-2 lead and the giant South African served it out to reach round two.
"It feels fantastic to be back. It's always difficult to miss tournaments, it's been a tough year, but there has been a lot of time and effort put in to get me back out there," said Anderson.
"It feels good now, and it held up well today with a long three-set match. I found a nice rhythm and on the grass that helps."
Daniil Medvedev was the first player through after the Russian world No 13 cruised past an error-strewn Fernando Verdasco 6-2, 6-4. French veteran Nicolas Mahut also progressed with a 6-3, 7-6 win over American Frances Tiafoe.
Among the players in action on Tuesday is Argentine Juan Martin Del Potro, who is making his first appearance at the tournament since 2016.
"I love this tournament," said Del Potro, who takes on Canadian Denis Shapovalov. "Unfortunately, I couldn't play these past few years but this time is a different moment for me. I want to be ready for Wimbledon and this tournament is a special one, so I am very excited to be here."
The world No 12 has been recovering from a knee injury and he is confident he is on the right track.
"I think I am getting better after my knee injury," he said. "I played well in Paris and now we move to a different surface, which I like. This tournament is also a good preparation to see how my body is, and my tennis level, also, and I think I will be fine."