Lehecka won easily in 63 minutes thanks to his strong groundstrokes and willingness to attack whenever he could.
Mertens had only ever won once before in the second round of the 2017 New Haven tournament, giving Kasatkina a 4-1 head-to-head advantage.
Swiatek was the top seed in Miami, and after receiving a first-round bye, she was scheduled to face American Claire Liu in the second round.
The 2021 finalist Andreescu won their first-round match in Rome last year after Raducanu withdrew due to a lower back injury while trailing 6-2, 2-1.
The 2019 Monte-Carlo semifinalist defeated Murray in their first career meeting to earn his first victory on a hard court this year.
“The time has come to give up the fiction that COVID vaccines remain a necessary tool to promote public health,” he said.
The US Tennis Association and the US Open's organisers both expressed their support for Djokovic's application to enter the country on Friday.
The 22-time Grand Slam champion tweeted about his withdrawals, which were anticipated but continue to raise doubts about when he will be able to play again.
The Spaniard's best Grand Slam run came during the 2021 US Open where he earned his place in the quarter-finals.
Alcaraz is the youngest Miami men's champion in the event's 37-year history, as well as the third-youngest ATP Masters 1000 champion overall.