Djokovic, gunning for a seventh Australian Open title, was also glad to be back, having missed the competitive edge.
Kerber, the other former Open victor in the women's tournament, won her 10th match in a row to start the season, beating Anna-Lena Friedsam 6-0, 6-4.
The Russian served a 15-month doping ban a year ago after testing positive for banned substance meldonium.
Despite a shaky finish on Rod Laver Arena on Thursday, Sharapova was relatively comfortable in a 6-1, 7-6 (7-4) second-round win to set up a match against 2016 champion Angelique Kerber. She celebrated by twirling, waving and blowing kisses to the crowd. "The third was up and down a little bit, but in general, it was a great performance", said the former six-time Australian Open champion. "I feel like I spend less energy but I'm more efficient".
But a series of wins at the Hopman Cup this month, coupled with ending a 27-tournament title drought by winning the Sydney International - her first since the US Open - has restored her confidence. But she made a meal of it against local hope Destanee Aiava.
Wawrinka was suspected to have a knee injury going into the tournament and clutched it throughout the contest. Gael is one of the best athletes in our sport.
Young didn't go out without a fight, however, saving four match points on his serve in the ninth game of the final set before Djokovic served out the match in the next game. "We know that", he said. He has been among the most vocal proponents for shortening the almost year-long tennis schedule to protect top players from injury.
No. 13 Sam Querrey restored some order for the US men with a 6-3, 6-4, 6-2 win over Feliciano Lopez, advancing to the second round. Two other USA contenders, No 8 Jack Sock and No 16 John Isner, were out on opening day.
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The top seeded men and women sailed through the first round in the two sessions on the second day that set records in attendance.
Sloane Stephens, Venus Williams and CoCo Vandeweghe were eliminated Monday.
No 17-seeded Keys, the runner-up at the US Open, had a 6-1, 7-5 win over Wang Qiang. "Shows how much depth we have in the women's game right now". The first two sets went extremely well, considering that I haven't played for six months.
The Wimbledon champion Garbiñe Muguruza fell out of the tournament after being beaten, 7-6 (1), 6-4, by Hsieh Su-wei.
"The first set was really good", Zverev said. "You use it as a measuring stick".
Wimbledon champion Garbine Mugurza, No. 6 Karolina Pliskova, No. 8 Caroline Garcia, No. 9 Johanna Konta, No. 16 Elena Vesnina, No. 18 Ash Barty, No. 26 Agnieszka Radwanska and No. 28 Mirjana Lucic-Baroni also advanced. In the victory, Sharapova won 82 percent of her first serve points and 46 percent of her second serve points.
Sharapova claimed a 6-1 7-6 victory over No 14 seed Anastasija Sevastova, the Latvian who had cut short her Grand Slam comeback at last year's US Open.