Venus Williams, Rafael Nadal and Andy Murray won their first round matches and advanced into the second round of Wimbledon.
On No 1 Court, Williams had to wait through a 33-minute rain delay to win her opening match. She had two match points before the rain came. Play was stopped with Williams leading 5-3, 40-40 in the second set.
When play resumed, Mertens held serve to make it 5-4, but Williams served out the match.
Defending champion Andy Murray also won his opening match, beating Alexander Bublik 6-1, 6-4, 6-2.
Murray entered the tournament after pulling out of a pair of exhibition matches because of a sore hip. "I was a bit nervous this morning," Murray said. "I hadn't been able to do as much as I would have liked in the buildup."
Petra Kvitova, a two-time Wimbledon champion who was injured in a knife attack at her home in December, beat Johanna Larsson of Sweden 6-3, 6-4 on Centre Court while Rafael Nadal defeated John Millman of Australia 6-1, 6-3, 6-2.
Second-seeded Simona Halep and No 17 Madison Keys also won. Halep, who lost in the French Open final, beat Marina Erakovic of New Zealand 6-4, 6-1. Keys defeated Nao Hibino of Japan 6-4, 6-2.
Keys, who is right-handed, was playing for the first time since having a second surgery on her left wrist.
"It's obviously not perfect," Keys said. "I haven't been able to do the repetitions that ideally I would have wanted to do, but it feels fine. Doesn't hurt anymore. That's really all I care about."
In the men's draw, Nick Kyrgios retired from his first-round match after losing the first two sets.
The 20th-seeded Australian was trailing Pierre-Hugues Herbert of France 6-3, 6-4 when he called for a trainer to work on his left hip. He then decided to stop the match. Kyrgios also retired with an injury from his first-round match at Queen’s, a Wimbledon warm-up tournament.
"I kind of knew I was in trouble. I have been feeling my hip ever since I fell over at Queen's. Never got it right," Kyrgios said. "I was doing everything I could to help it, but just not enough time."
Two-time semifinalist Jo-Wilfried Tsonga also reached the second round. The 12th-seeded Frenchman beat Cameron Norrie of Britain 6-3, 6-2, 6-2.
Ninth-seeded Kei Nishikori and 24th-seeded Sam Querrey also advanced.
(With inputs from AP.)
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