Grigor Dimitrov scraped off the rust in his game to overcome Japan's Yoshihito Nishioka 6-3, 6-4 and move into the second round of the Brisbane International on Monday.
The Bulgarian, who struggled with an ankle injury towards the end of last season, saved six out of seven break points to down his unseeded opponent in the season-opening tournament.
Up next for the 2017 champion is Australia's John Millman or American Tennys Sandgren.
The big-serving Milos Raonic, another former world number three who slid down the rankings last season to sit one place above Dimitrov, fired 18 aces in his 6-0, 6-3 win over Slovenia's Aljaz Bedene.
Canadian Raonic, the 2016 winner in Brisbane, next takes on qualifier Miomir Kecmanovic.
Among the women, 2017 winner Karolina Pliskova rallied from a set and a break down to beat Yulia Putintseva 4-6, 6-3, 6-4.
Wildcard Kimberly Birrell fought back from 3-5 down in the decider to stun Russia's world number 10 Daria Kasatkina 5-7, 6-4, 7-6(3) for her first win over a top-10 player.
Fellow Australian Destanee Aiava produced another upset, beating Kristina Mladenovic 6-3, 7-6(2) to set up a second round clash with US Open champion Naomi Osaka.
Five-time Grand Slam champion Maria Sharapova had a tough start to her 2019 tennis season on Monday at the Shenzhen Open, edging out Switzerland's Timea Bacsinszky 6-2, 7-6 (7/3).
In a battle of former top 10 players, Sharapova's serve faltered early in the match but she regained composure to take the first set, cheered on by the spirited crowd at the Shenzhen Longgang Sports Center.
Bacsinszky put up a strong fight in the second set, claiming the first two games and forcing Sharapova to step up her game. The Russian powered through to take the match in one hour 42 minutes.
Meanwhile, France's Caroline Garcia lost in straight sets to Serbia's Ivana Jorovic 6-4, 6-2.
Former Wimbledon finalist Eugenie Bouchard launched her comeback year in winning form as she cruised through her first round match in the WTA Auckland Classic on Monday.
Olympic champion Monica Puig also sailed through to the second round on a day in which none of the seeded players appeared.
Former world number five Bouchard has earmarked 2019 as her comeback year after sliding down rankings following a standout 2014 when she made the Wimbledon final and was a semifinalist in Melbourne and Paris.
Now ranked 87th, the Canadian proved too strong for the 88th ranked Madison Brengle, taking just over an hour to beat the American 6-3, 6-3.
The 24-year-old mixed glimpses of her previous best with some unforced errors but overall gave herself a pass mark.
Puig was untroubled in her 6-3, 6-2 win over American wildcard Bethanie Mattek-Sands while 2017 champion Lauren Davis advanced with a 6-2, 6-2 win over Spain's Lara Arruabarrena.