Marin Cilic followed up his weekend indoor title by beating Lukas Rosol 6-2, 6-2 on Tuesday while former champion Mikhail Youzhny was bundled out at the Rotterdam World Tennis.
The unseeded Cilic came to the Dutch port city fresh from a fourth title at home in Zagreb where he beat Tommy Haas. The former world number nine next plays fifth seed Jo-Wilfried Tsonga.
Youzhny, seeded seventh, went down double breaks in both sets as the 2007 winner fell to Dutchman Igor Sijsling 6-2, 6-2.
Youzhny's 2014 loss streak is now entering a second month after he won his only match of the season a month ago in the Australian Open first round.
Latvian Ernests Gulbis earned a second-round spot as he beat Uzbek Denis Istomin 6-4, 7-6 (7/4) in 90 minutes.
Cilic, who served a three-month ban for a technical anti-doping infraction last season after he inadvertently ingested a banned substance through the use of over-the-counter glucose tablets, currently stands 37th, but is making steady progress in his return.
"At the start of the season, I though it might go quicker," said Cilic, now coached by former Croatian star Goran Ivanisevic. "But my game has been a bit up and down, oscillating.
"I've still not played a lot of matches," said the player who returned to the ATP last November at Paris Bercy.
"I'm still not used to some match situations, some balls come differently than in training, But I'm getting back in the groove now and playing much better to that (his absence) is all behind me now."
Cilic, with ten ATP titles, never lost control against the 50th-ranked Czech Rosol, with the winner sending down a modest eight aces but breaking four times in the one-hour victory.
"Overall, I played solidly, I didn't have too many mistakes. In the first couple of games I had to get used to conditions. I was finally feeling the ball. I mixed it up well and made him play.
"My serve was also working pretty well, it was a solid match."
Meanwhile, Czech third seed Tomas Berdych has admitted to gaining confidence about his Grand Slam title chances after watching Switzerland's Stanislas Wawrinka win the Australian Open.
Number seven Berdych was himself a semi-final victim of Wawrinka last month in Melbourne, with the Swiss winning the title over Rafael Nadal.
"Stan showed us that it is possible to win a Grand Slam," said Berdych, who meets either Nicolas Mahut or Gilles Simon, both of France, in the second round. "Grand Slams are difficult to win, but Stan showed it's possible to beat the top guys.
"If you work hard, then everything can click together and you can make it in the end. It isn't only the right of two or three guys (Roger Federer, Nadal, Novak Djokovic and Andy Murray, the so-called Big Four).
"Stan's win showed that the top 10 is also extremely tough. I just need to work harder, stay patient and hopefully the results will come," added the 2010 Wimbledon finalist.