While all went according to the form book on the women's side of the draw, the opening day of action threw up plenty of surprises for the men.
Almagro was joined on the scrapheap by number 14 Fabio Fognini of Italy and 16th seed Jerzey Janowicz of Poland, all of whom fell at the first hurdle.
Almagro was stopped 7-6 (3) 6-4 by Dimitrov while Czech Radek Stepanek dumped Fognini 6-2 6-4 and wildcard American James Blake eliminated Janowicz 6-1 7-5.
An impressive line-up in Cincinnati - the final warm-up tournament ahead of the US Open - features world number one Novak Djokovic, Wimbledon champion Andy Murray, Rafa Nadal, also a winner in Canada on Sunday, and Roger Federer.
With the top seeds receiving first round byes some of the bigger names will begin to ease into the action on Tuesday with Federer, seeking to add to his five Cincinnati titles, opening his account against Germany's Philipp Kohlschreiber.
Cincinnati Masters results
James Blake (U.S.) beat 16-Jerzy Janowicz (Poland) 6-1 7-5
David Goffin (Belgium) beat Mackenzie McDonald (U.S.) 6-1 6-1
Dmitry Tursunov (Russia) beat Martin Klizan (Slovakia) 4-6 7-6(6) 6-4
Janko Tipsarevic (Serbia) beat Sam Querrey (U.S.) 6-7(5) 6-3 6-4
Radek Stepanek (Czech Republic) beat 14-Fabio Fognini (Italy) 6-2 6-4
Juan Monaco (Argentina) beat Jurgen Melzer (Austria) 6-3 6-2
Jeremy Chardy (France) beat Adrian Mannarino (France) 6-4 6-3
Grigor Dimitrov (Bulgaria) beat 13-Nicolas Almagro (Spain) 7-6(3) 6-4
Marcel Granollers (Spain) beat Lukas Rosol (Czech Republic) 5-7 6-4 6-0
Philipp Kohlschreiber (Germany) beat Mardy Fish (U.S.) 7-5 6-2
Brian Baker (U.S.) beat Denis Istomin (Uzbekistan) 7-5 6-3