DELRAY BEACH, Fla. -- Marinko Matosevic ousted No. 2 seed John Isner 6-4, 6-4 in the first round of the Delray Beach Open on Tuesday.

Matosevic had never beaten Isner in three previous meetings, all in 2014.

The 29-year-old Isner, a semifinalist in Delray the past three years, struggled throughout the 75-minute match, slapping forehands into the middle of the net and dropping serve to go down 4-3 in the first set and then again in the first game of the second set.

Matosevic, who reached his only ATP final here in 2012, saved a break point in the eighth game of the second set. Isner then hit three straight aces to save a break point and hold serve for 4-5.

But Matosevic held at love for the win when Isner netted yet another backhand return.

"I felt like I was the better player out there," said Matosevic who, after winning match point, yelled "second serve," referring to his improving second delivery.

"My second serve was different in our three previous matches and I felt like he was able to take advantage of it."

"I just didn't play well, didn't do anything well," said Isner, who hit 16 aces but failed to capitalize on three break opportunities, including one when Matosevic was serving at 4-3 in the second set and he sent a backhand crosscourt passing shot into the net.

"I'm disappointed, but I'm just not good enough to not play well against this group of guys. It's all between the ears right now. It's not for lack of effort."

Sam Querrey felt good when he arrived in Delray Beach.

He was in top physical shape and confident after a semifinal showing in Memphis last week. But with one false move during his first match at the Delray Beach Open, Querrey injured his back and retired while leading Alejandro Gonzalez of Colombia 6-3, 1-2.

The sixth-seeded Querrey was serving at deuce when he bent low for an awkward backhand volley, missed the shot and then clutched his lower back. After attempting to stretch for a moment, Querrey asked for the trainer and left the court for treatment. Nearly nine minutes later, the 27-year-old American, a semifinalist here in 2008, returned, shook his opponent's hand and conceded the match.

"It was just one sudden movement and I jerked my back," said Querrey, who will have an MRI before deciding whether he can represent the U.S. in the first round of the Davis Cup against Great Britain in Glasgow, Scotland, on March 6-8. "I've had this before and it's gotten better in a week. I'm just bummed because I played so well last week."

In other matches, top-seeded Kevin Anderson earned a 6-3, 6-3 win over qualifier John-Patrick Smith, an Australian who honed his game at the University of Tennessee.

Anderson, seeded No. 1 for only the second time in his career, is 14-4 in Delray Beach, having won the tournament in 2012 and reaching the final here last year. Last week he was runner-up in Memphis to Kei Nishikori.

"I'm feeling pretty confident, just sticking with the basics," said Anderson, a South African who makes his home just north of Delray Beach.

Anderson arrived late in Delray when his flight from Memphis was delayed and then cancelled due to inclement weather, forcing him and his coach to drive through the snow to Birmingham, Alabama, where they caught a flight to Miami, arriving less than 24 hours before his first-round match.

Three Americans moved into the second round. Tim Smyczek beat 17-year-old wild card Stefan Kozlov 7-5, 6-2, while Donald Young, another semifinalist in Memphis, defeated Croatia's Ivan Dodig 7-5, 7-5. Seventh-seeded Steve Johnson advanced when Mikhail Kukushkin of Kazakhstan retired because of illness while down 6-3, 4-1.

"I knew coming in that (Kozlov) would make it an uncomfortable match," said the 27-year-old Smyczek, who led Rafael Nadal by two sets to one in the second round of the Australian Open in January before losing 7-5 in the fifth set. "He's good at taking you out of your game. But I was prepared for his style and felt the ball really well."

Three of the four teens in the draw also won their matches. Seventeen-year-old Russian Andrey Rublev, a wild card ranked No. 370, won the first ATP match of his career with a 6-4, 1-6, 6-3 victory over veteran Dudi Sela of Israel.

Two qualifiers, 18-year-old Thanasi Kokkinakis from Australia and 19-year-old Yoshihito Nishioka of Japan, also moved on. Kokkinakis beat Serbia's Filip Krajinovic 6-2, 6-3, and Nishioka defeated Dutchman Igor Sijsling 7-5, 6-1. Rublev was last year's No. 1-ranked junior in the world.