Delray Beach, FL, USA, 18 February 2015 | tennis.com.au

Marinko Matosevic has recorded one of his best-ever victories with a straight-sets upset win over No.2 seed John Isner at Delray Beach.

The Victorian beat the 19th-ranked American 6-4 6-4 in one hour and 15 minutes to set up a second-round match against Japanese qualifier Yoshihito Nishioka.

“I felt like I was the better player,” Matosevic said.

“I had so many chances in the second set and I thought I returned great tonight, returned his serve well and really happy with the win.”

Matosevic has a history of performing well at the ATP 250-level event in Florida; in 2012 he enjoyed a breakthrough performance on the ATP World Tour when he reached the final as a qualifier.

It was a victory that put him on the path to the world’s top 100, where he has since remained for much of the past three years.

“It was great memories. Same kind of thing (as beating Isner first up in 2015), I beat Karlovic, a big server, in the first round, then went on to make the final,” Matosevic said of his 2012 run.

“Hopefully I’d love to win the tournament where it all started for me.”

His defeat of Isner is his sixth win over a top 20 opponent; his best came against then-13th-ranked Milos Raonic in Washington DC in 2013. Matosevic also defeated then-No.12 Tommy Haas in Montreal that same year, albeit when the German retired with injury.

On Tuesday night in Delray Beach, Matosevic survived 16 aces from the big-serving Isner and managed to score five break point opportunities, two of which he converted – one in each set.

He also saved all three break points that he faced.

It was Matosevic’s first victory over the American in four career meetings; Isner’s three previous victories all came in 2014.

The Aussie said the win was a sign that he was returning to health and form after a long period of niggling ailments.

“I haven’t been able to play the tennis I want to play … Little nagging injuries in my lower back which affected my pre-season, and I’ve had bursitis for a year and a half and I strained my right hip flexor in Sydney,” he said.

“My body is starting to feel better and better, but it’s been a tough last six months.”

Earlier in the night session, top seed Kevin Anderson was too strong for Matosevic’s compatriot John-Patrick Smith.

The South African bested the Aussie qualifier 6-3 6-3 to advance to the second round.

Matosevic joins fellow Australians Bernard Tomic and Thanasi Kokkinakis in the second round, while Sam Groth was unable to join the trio, falling in three sets to Taiwanese player Yen-Hsun Lu.

> READ MORE: Kokkinakis, Tomic win at Delray Beach