Cincinnati - Stan Wawrinka won a first-round battle as he eliminated 2017 Cincinnati Masters champion Grigor Dimitrov 5-7, 6-4, 7-6 (7/4) on Tuesday.
Under leaden skies after an overnight thunderstorm, Dimitrov broke in the 12th game of the first set to take the lead.
The 33-year-old Swiss fought back with the only break in the second.
He then raced into a 5-1 lead in the final set but wasted two match points in the eighth game and was broken twice while serving for victory.
Wawrinka had to hold off his Bulgarian opponent in the final-set tiebreaker before squeezing through in more than two and a half hours with his 10th ace.
The loss left Dimitrov with just two victories out of six matches played since Roland Garros and four consecutive losses to Wawrinka, including last week's opening round in Montreal.
The Swiss who played a 2012 semi-final here, will face Russian qualifier Andrey Rublev, a 6-7 (4/7), 6-4, 6-2 winner over 15th seed Nikoloz Basilashvili from Georgia.
In other opening-round matches, Belgian David Goffin beat Taylor Fritz of the United States 6-4, 4-6, 6-4 and France's Benoit Paire advanced 6-4 as Spaniard Fernando Verdasco retired.
In women's play, 2013 champion Victoria Azarenka reached the second round when Belinda Bencic retired injured, trailing 6-4, 1-0.
Results on Tuesday in the WTA/ATP Cincinnati Masters (x-denotes seeding):
Stanislas Wawrinka (SUI) bt Grigor Dimitrov (BUL) 5-7, 6-4, 7-6 (7/4)
Andrey Rublev (RUS) bt Nikoloz Basilashvili (GEO x15) 6-7 (4/7), 6-4, 6-2
Benoit Paire (FRA) bt Fernando Verdasco (ESP) 6-4, 0-0 abandoned
Frances Tiafoe (USA) bt Gael Monfils (FRA) 7-6 (9/7), 6-3
David Goffin (BEL x16) bt Taylor Fritz (USA) 6-4, 4-6, 6-4
Anett Kontaveit (EST) bt Angelique Kerber (GER x13) 7-6 (9/7) 6-2
Saisai Zheng (CHN) bt Viktoria Kuzmova (SVK) 6-3, 6-1
Donna Vekic (CRO) bt Katerina Siniakova (CZE) 6-4, 6-3
Victoria Azarenka (BLR) bt Belinda Bencic (SUI x12) 6-4, 1-0 abandoned
Ekaterina Alexandrova (RUS) bt Carla Suarez (ESP) 6-4, 6-1