Alex de Minaur has opened his campaign at the ATP's Acapulco Open with a highly impressive straight-sets victory.

The Australian eighth seed, who boomed down four aces and struck 25 winners, needed just 77 minutes to seal a 6-1 6-2 win over local wildcard Rodrigo Pacheco Mendez.

De Minaur improved to 8-4 on the season, reaching consecutive quarter-finals in Rotterdam and Marseille before arriving in Mexico.

"It's never easy, especially first match at a new tournament," de Minaur said.

"These conditions are tricky. The opponent was tricky. I'm very happy with the performance, happy with the win, Hopefully we can build on that and go better and bigger for the next round."

The world No.22 wasted no time in taking charge, breaking the 17-year-old in the opening game and then again in the fifth and seventh games of the first set.

Another two breaks of serve by the 24-year-old Sydneysider in the second set up a last-16 clash with qualifier Jacopo Berrettini, after the Italian progressed via a walkover when leading Oscar Otte of Germany 3-6 7-6 (7-3) 2-1.

"I've been watching him (Berrettini) this tournament," de Minaur said of the world No.842.

"He's been playing some great tennis, obviously played way above his ranking. He's going to be confident, he's going to be a very tough match and I'm ready for a battle."

American Brandon Nakashima is through to the round of 16 having held off another German in Daniel Altmaier, 6-3 3-6 6-1, while compatriot Mackenzie McDonald triumphed 6-4 6-2 over Luciano Darderi.

Earlier, Slovakia's Alex Molcan was forced to retire when trailing 6-0 1-0 against Berrettini's older brother Matteo, while Swede Elias Ymer beat French veteran Adrian Mannarino 5-7 6-3 6-4.

At the ATP event in Santiago, former world No.3 and 2020 US Open champion Dominic Thiem continued to struggle on clay as he works his way back from right wrist and abdominal injuries.

The 29-year-old Austrian lost 6-2 7-6 (7-2) to Christian Garin in the first round, the Chilean advancing to the last 16 where he'll face Argentine Sebastian Baez.

Thiem has slipped down the rankings to 99th after claiming just one singles victory on tour this year -- against Molcan at the Argentina Open earlier in February.

In other results, Thiago Monteiro beat fifth-seeded Albert Ramos-Vinolas 7-5 4-6 7-5 to move into a match against Marco Cecchinato, who beat Facundo Bagnis 6-2 3-6 7-6 (7-3).

With AP.