The analysis was dead on. Groth was broken three times in the opening set and never recovered.
Murray, the second-ranked Scot, won the first nine games of the match and rolled to a 6-0, 6-4, 6-1 win to reach the third round at the Australian Open.
Murray has faced little resistance through two rounds. He showed some frustration after losing an early break in the second set but quickly regrouped, breaking Groth back in the 10th and final game of that frame to take a commanding lead.
On match point, Murray slugged an ace out wide, his 10th.
Groth, the fastest server on tour, had just six aces and was broken seven times.
Murray played a crisp match all around. He had 35 winners and committed only 10 unforced errors. He improved his record against Australian players to 17-0, though it should be noted he lost to Nick Kyrgios a couple of weeks ago in the Hopman Cup, an exhibition.
"[Groth] didn't start the match off serving that well, which helped," Murray said. "Because I was returning well, that maybe put some more pressure on him. He didn't really start serving well or how well he can serve until maybe, you know, middle of the second set. Then, you know, made it tougher there. But, yeah, I returned good and passed and lobbed well. That's what I needed to do today to get the win."
"No, it wasn't easy at all," Isner, the No. 10 seed, said in his on-court interview. "It was a very tough match. I'm just happy to get through. It's always a pleasure to play on this court."
Predictably, Isner's serve carried him into the round of 32 for the fifth time Down Under. He slugged 20 aces and didn't drop serve or face a break point. Through two matches Isner has dropped only 19 first-serve points.
Isner finished 2015 at No. 11 in the world, the highest year-end ranking of his career. While he won a title in Atlanta, the knock on him has been his inability to break through in majors. Only once, in the 2011 US Open, has he reached a Grand Slam quarterfinal.
Isner, who will next face Feliciano Lopez, looks to reach the round of 16 in Australia for the first time in six years.
"He actually beat me on this court a few years ago," Isner said of Lopez. "We have been on tour for eight years together. He's a big lefty server and likes fast courts. It will be a tough match."
Through two rounds, Isner has yet to drop a set, an encouraging sign from a player who has a long, mostly dubious, history of marathon matches. Isner spoke of the efficient work he did to get himself into shape for Australia.
"I took care of myself," he said. "I worked hard. I worked smart. Just taking care of myself. I feel good."
Coming into the tournament, Isner also said he is going to channel the mantra of the Carolina Panthers and "keep pounding."
Nicolas Mahut, Isner's opponent in the longest match in tennis history at the 2010 Wimbledon Championships, wasn't as lucky. He fell to French countryman Gael Monfils 7-5, 6-4, 6-1 on Thursday. For Monfils, who slugged 50 winners against only 10 unforced errors, this is the eighth time he has reached the third round.
In late November, Murray led his Great Britain team to its first Davis Cup title in 79 years. Since then, he has talked openly of how much he relished the moment, how it put him at ease, especially compared with his Wimbledon championship three years ago, which Murray described as more relief than joy.
Murray, who is 41-10 in his career Down Under, hopes to become the first player to win a Grand Slam after losing four finals at that event.
He sits in a comfortable section of the draw. On Saturday, he'll take on No. 32 Joao Sousa, and assuming Murray prevails, he could face another Aussie, Bernard Tomic, the 16th seed, in the fourth round and then Isner in the quarterfinals.
Tomic advanced into the third round with a 6-4, 6-2, 6-7 (5), 7-5 win over Simone Bolelli on Margaret Court Arena.
No. 14 Milos Raonic had 24 aces as he fended off Tommy Robredo 7-6 (6), 7-6 (5), 7-5 in a match containing one service break. No. 21 Viktor Troicki and No. 31 Steve Johnson also advanced, while No. 25 Jack Sock and No. 30 Jeremy Chardy were ousted.
Information from The Associated Press was used in this report.