Jo-Wilfried Tsonga remained undefeated at the Austrian Open as the top seed clinched a 7-5, 1-6, 6-3 defeat of German Daniel Brands to reach the quarter-finals on Thursday.

"So far, I'm undefeated here," joked Tsonga, who won the title in his only previous appearance in 2011.

The Frenchman suffered a minor lapse in the second set as Brands -- a winner over Roger Federer on clay at Gstaad in July -- fought back to level the match at a set each.

But Tsonga, in contention for one of the remaining spots in the season-ending ATP World Tour Finals next month in London, quickly regained his superiority to complete victory in 93 minutes with seven aces and two breaks of serve.

"It was a tough match to start with," said Tsonga, who suffered a knee injury in the summer and is competing in his fourth event over the past five weeks as he aims to qualify for the season wrap-up.

"It was not easy physically and Brands is a difficult opponent. But the important thing is that I won."

Former champion Tommy Haas saw off qualifier Miloslav Mecir Jr 7-5, 7-6 (10/8) to reach the quarter-finals.

The 25-year-old Slovak qualifier is the son of former number four Miloslav Mecir, the 1988 Olympic champion who also reached the 1986 US Open and 1989 Australian Open finals.

Second seed Haas, 35, is playing the Vienna event for the 11th time, having won the title in 2001 and now stands at 12-9 at the Stadthalle.

But the win, in just under two hours, was far from simple for Haas, who withdrew last week in Shanghai before his third-round match with back pain.

The German had to save a set point in the opener before finally winning 7-5 in three quarters of an hour.

Haas needed to battle to close out the match, missing on two match points through a double-fault and a forehand error.

The 240th-ranked Mecir, whose opening-round win was his first at the ATP level, had two set points before Haas finished off the job on his third match point.

Mecir is the second son of a former top 10 player to win an ATP match in 2013. In July it happened to Emilio Gomez, whose father is 1990 Roland Garros champion Andres Gomez, at the tournament in Quito.

The younger Mecir told the ATP that he takes after his smooth-hitting father on court.

“I've been told that I have a pretty similar playing style to my father. I can adjust my style dependent on who I play. I can take the ball early, especially on hard courts, and I also have the ability to finish points without my opponent making a mistake," he explained.

The Slovak came close to giving up the game two years ago due to injury and began a college IT course which he has continued.

“I had inflammation of the elbow ligaments and I did not play for four months. When I was injured, I thought it was a good idea to not only play tennis, but also use my mind,” he added.

Haas was happy to reach the next round.

"I was very impressed," said the German of Mecir. "I had heard his name before and seen it in draws. I actually played Karol Kucera when his father was coaching him. Mecir has a similar game to his dad, he played well."

Italian third seed Fabio Fognini began with a victory over Pole Lukas Kubot 7-5, 6-2 while Robin Haase defeated seventh seed Vasek Pospisil 7-6 (7/3), 6-7 (5/7), 6-2.