Andy Murray made hard work of his first grand slam match as world number one, while Stan Wawrinka fought back from a break down in the final set to book his place in the second round of the Australian Open
Murray shouted "wake up!" during one of many frustrating moments on Rod Laver Arena but a dozy display proved sufficient as the Scot sealed a 7-5 7-6 (7/5) 6-2 victory.
He will now face Russian teenager Andrey Rublev, the world number 152 who had earlier registered a surprise win over Yen-Hsun Lu.
The 29-year-old knows he will have to play far better than this if he is to lift the trophy a week on Sunday, after a scruffy performance that included 27 unforced errors, three conceded breaks of serve and a tendency to adopt the kind of careful, cautious tennis that will run him into trouble against more proficient opponents.
Marchenko, ranked 95th in the world and wearing his cap back to front, certainly offered a stern opening test, his exuberant shot-making and refusal to give in courting much support from an appreciative Australian crowd.
But baking hot conditions, with temperatures reaching 32 degrees Celsius, took their toll on the Ukrainian, along with Murray's superior experience under pressure, which ultimately proved decisive.
Marchenko's early break felt like little more than Murray blowing out the cobwebs, particularly when the Scot broke back before taking a 5-3 lead.
His shots, however, were loose and his energy lacking, and Marchenko came again, deservedly breaking a second time after picking up a deft forehand volley.
Murray did just enough to edge the opening set, when Marchenko over-reached on a forehand, but seeds of confidence had been sewn in his opponent, who seized another break early in the second.
Frustrated, Murray bellowed to himself, and perhaps to his box, "Wake up!" before adding, "This is a grand slam guys, come on".
Murray often needs a dose of agitation to jolt himself into action, however, and so it proved as he broke back for 4-4 before a brilliant top-spin lob, clipped on the run, put him 6-5 ahead.
Marchenko, who had missed a break point in the previous game, extended Murray to a tie-break but he wavered under pressure as three missed forehands helped Murray to a 4-1 lead.
There was no way back as Murray converted his third set point to move two sets clear and while Marchenko took a toilet break, perhaps to refocus, the rest of the contest was one-way traffic.
Murray broke twice to storm into a 5-1 lead in the third set and - when a final Marchenko forehand drifted long - his progress was confirmed in two hours and 47 minutes.
Switzerland's Stan Wawrinka survived a scare from Slovakia's Martin Kliza, fighting back from a break down in the final set to move into the Australian Open second round with a 4-6 6-4 7-5 4-6 6-4 victory.
It took the world number four more than three hours to knock out his opponent to cries of "Allez Stan" from the crowd after he had struggled early on to cope with Klizan's signature delicate dropshots.
Wawrinka, a three-times grand slam champion, gradually found the range on his lethal forehand, however, and he has never lost in the Australian Open first round in 12 visits to Melbourne Park.
He will face American Steve Johnson in the second round after the world number 30 beat Argentina's Frederico Delbonis 6-3 6-4 6-4.
Swiss 17-times grand slam champion Roger Federer, seeded 17th after six months out of the game to recover from a knee injury, returned to the court with a 7-5 3-6 6-2 6-2 win over Austrian qualifier Juergen Melzer in the late match at Rod Laver Arena.
Australian Nick Kyrgios did little wrong against Gastao Elias, beating the Portuguese 6-1 6-2 6-2 in an hour and 25 minutes in the first round, and the 14th seed will now face Italian Andreas Seppi in the next round.
Former Australian Open finalist Jo-Wilfried Tsonga books his spot in the second round with a four-set win over Brazil's Thiago Monteiro to set up a showdown with Serbia's Dusan Lajovic.
Kazakhstan's 19-year-old Alexander Bublik scored a major upset against Lucas Pouille, beating the French 16th seed 6-0 3-6 6-3 6-4 in an hour and 43 minutes to advance to the second round, where he will face Tunisia's Malek Jaziri.
Spanish 10th seed Carla Suarez Navarro made quick work of Jana Cepelova, beating the Slovak 6-2 6-2 in just over an hour in the first round.
Navarro will face Romania's Sorana Cirstea in the next round.
