WINSTON-SALEM, N.C. (AP) Defending champion Lukas Rosol of the Czech Republic opened with a 6-4, 7-6 (4) victory Monday over Latvia's Ernests Gulbis in the first round of the Winston-Salem Open.
The 6-foot-5 Rosol served 14 aces and took advantage of a break point in the seventh game of the first set to improve to 3-1 all-time against Gulbis, and 2-1 this year, in the final tournament before next week's U.S. Open.
''It's not easy to defend the title,'' Rosol said. ''It was not easy, because two weeks ago I lost to (Gulbis) at Montreal in the second round. So I'm happy to continue winning here. I was trying concentrate on every point.
''Ernests was starting to serve very well in the second set, so that made things tricky. But I was happy to finish it in the tiebreak.''
Rosol, who was ranked as high as 26th in the world in 2014 before slipping to his current No. 86 spot, will face Poland's Jerzy Janowicz on Tuesday in a rematch of last year's Winston-Salem Open final.
In that match, Janowicz won the opening set 6-3, but Rosol fought back to take the next two sets 7-6 (7-3), 7-5 for his second career title.
''Jerzy's playing good tennis this month,'' Rosol said. ''It's not easy always against him. It'll be similar to Ernests because he's serving very well as well. So I'll have to wait for my chances, and see if Jerzy will give me those or not.
''I think maybe I have a little advantage because I've finished a match, while he hasn't played a match.''
In other matches at the Wake Forest Tennis Center, which had a 2-hour, 15-minute rain delay, 19-year-old Thanasi Kokkinakis of Australia upset Ukraine's Alexandr Dolgopolov 7-6 (1), 7-6 (5).
Kokkinakis, ranked 70th in the world, and the 39th-ranked Dolgopolov combined for 32 aces. But it was Kokkinakis' play in the tiebreakers that lifted him to his first career win over the 26-year-old Dolgopolov.
''He was feeling the ball well, hitting it really hard and putting me on my toes,'' said Kokkinakis, who will face Russia's Teymuraz Gabashvili in the second round.
''I just tried to stick with him. Luckily, my serve helped me a lot at the end. I was able to play the big points when it mattered.''
Also Monday, 17-year-old qualifier Frances Tiafoe overcame 30 aces by Australia's James Duckworth to win 3-6, 7-6 (4), 7-6 (3), and 37-year-old Tommy Haas of Germany, on the comeback from shoulder surgery, lost to Spain's Pablo Carreno Busta 6-3, 6-4. Uzbekistan's Denis Istomin downed Donald Young of the U.S. 6-3, 6-3.
Tiafoe, who will play Brazil's Thomaz Bellucci in the second round on Tuesday, became the youngest American to win his first ATP World Tour match since then-15-year-old Ryan Harrison won in Houston, Texas, in April 2008.