Dominic Thiem had no issue with Alexander Bublik's underarm serves but found his unorthodox game a real problem before battling his way into the third round of the French Open.

Bublik likes to use his full repertoire of trick shots during matches and pulled out the underarm serve on three occasions before going down 6-3 6-7 (6/8) 6-3 7-5 to the fourth seed.

It has been an increasingly familiar sight on the men's tour since Nick Kyrgios used it against Rafael Nadal in Acapulco in February, with the Spaniard later accusing his opponent of lacking respect.

Thiem said: "I expected it. To be honest, it's a good choice against players like us who are that far behind the baseline. There is nothing bad about it."

Thiem, who reached his first grand slam final here last year, is seen as one of the only men who could challenge Nadal and Novak Djokovic, but he has been made to work very hard through his first two rounds.

After being taken to four sets by Tommy Paul in round one, the Austrian trailed Bublik by a break in the fourth set but finished strongly.

He said: "It was a tough match-up for me because there were almost no rallies. He gave me no rhythm at all. He did it very well, actually. I was playing really bad once I got into the rally, which made it even more difficult.

"And generally it was a strange match, I think. Somehow the fire never got there. Probably for the spectators it was nice to watch because he played so different, but for me it was very tough. And at the end, I'm only happy that I'm through this in four sets."

Djokovic had no problems brushing aside Swiss lucky loser Henri Laaksonen 6-1 6-4 6-3.

It appeared the match might not even be that close when the top seed allowed his opponent just five points in the first five games but Laaksonen did at least dig in to make it somewhat competitive.

"I played as much as I needed to play to win in straight sets," said Djokovic. "I didn't spend too much time on the court. So all is going in the right direction."

In the next round he will face Italian qualifier Salvatore Caruso, who surprisingly beat 26th seed Gilles Simon for the loss of just seven games.

After another five-set match in the first round, fifth seed Alexander Zverev won in straight sets for only the second time at Roland Garros, beating Mikael Ymer 6-1 6-3 7-6 (7/3).

"In important moments I'm finally being very aggressive and stepping up," said Zverev, who next plays 30th seed Dusan Lajovic. "I think I'm starting to play really well."

Eighth seed Juan Martin Del Potro was given a real examination by Japan's Yoshihito Nishioka but came through 5-7 6-4 6-2 6-7 (5/7) 6-2 after three hours and 46 minutes.

Home favourite Gael Monfils beat fellow Frenchman Adrian Mannarino 6-3 6-4 6-4 and will take on another compatriot next in wild card Antoine Hoang, who upset 23rd seed Fernando Verdasco to reach the third round at his first grand slam.

Last year's quarter-finalist Diego Schwartzman was another seed to fall, the Argentinian losing in four sets to countryman Leonardo Mayer.