Top seed John Isner captured a 6-3, 6-4 win over fellow American Ryan Sweeting on Wednesday to move into the Delray Beach International Tennis Championships quarterfinals.
The 11th-ranked Isner struggled through his first round win over wildcard recipient Jesse Levine on Tuesday because of the humidity, but was in better shape playing Sweeting.
"Whenever, in the past, when I've played tournaments in humidity I've always struggled because I hadn't acclimated yet," Isner said. "But tonight I felt so much better than I did yesterday.
"I'm still not all the way there. I could feel better out there but it was so much better."
Isner broke Sweeting's serve twice in the match-in the second game of the first set and the fifth game of the second set-which was all he needed to take the win.
Sweeting was unable to capitalize on five break points when Isner kicked his potent serve into overdrive to save the points.
"A lot of times for me to be successful I'm going to have to have my serve and I did today when I needed it," Isner said. "I served well, went for my second serve, wasn't going for broke, but I hit my spots and it got me out of a couple of jams."
Isner will face eighth seed Bernard Tomic in the quarterfinals on Friday. The 37th-ranked Tomic scored a 6-1, 7-5 win over American qualifier Tim Smyczek, ranked No. 287, to move ahead in the draw.
Australian qualifier Marinko Matosevic beat No. 6 seed Alex Bogomolov Jr. of Russia 4-6, 6-1, 6-3 to reach his first career ATP quarterfinal.
The 173rd-ranked Matosevic had 11 aces and won five of eight break points.
"It was a tough match and I'm happy to get through and make my first quarterfinal," said Matosevic, who next faces 2010 Delray Beach champion Ernests Gulbis. "I knew his game pretty well and he reaches a lot of balls, so I just had to keep staying aggressive.
"I played the big points well."
The 26-year-old has spent most of his career playing on the Challenger circuit.
Bogomolov had the best season of his career in 2011, starting the year at No. 166 and finishing at No. 34. Fellow players voted him the year's most improved player.
Bogomolov struggled throughout the match, including in his victorious first set.
Gulbis beat Steve Darcis of Belgium 6-7 (6), 6-1, 6-3 to reach his first quarterfinal of the season. The Latvian won 40 of 45 points on his first serve and fired 10 aces.
"Today was tough because I couldn't get my timing right in the first set," said Gulbis, who said the wind made it tough to play. "From one side it was much easier to get your serve going than the other side.
"I didn't play my best tennis today but I'm happy I won the match because I was fighting."