Mason - World number one Rafa Nadal moved into the third round of the Cincinnati Masters with a 6-2, 7-5 victory over American Taylor Dent on Wednesday.
In the first real upset of the tournament, Nadal's compatriot Fernando Verdasco, seeded eighth, lost to 36th-ranked American Mardy Fish.
Journeyman Dent, ranked 83rd in the world, struggled in the first set but the 29-year-old's serve improved noticeably in the second and he threatened to make life complicated for Nadal when he broke and went 4-2 up in the second.
The Spaniard swiftly broke back and then broke again to go 6-5 up before serving out for the match.
Nadal will face Julien Benneteau in the next round after the Frenchman defeated Switzerland's Stanislas Wawrinka 6-4, 6-2.
Fish, who has credited his recent surge in form to weight loss, entered the tournament on a wildcard but played some inspired, attacking tennis to beat Verdasco in two tie-breaks.
Serbian world number three Novak Djokovic also progressed in straight sets, defeating compatriot Viktor Troicki 6-3, 7-5.
Round two results:
Rafa Nadal (ESP) bt Taylor Dent (USA) 6-2, 7-5
Mardy Fish (USA) bt Fernando Verdasco (ESP) 7-6, 7-6
Novak Djokovic (SER) bt Viktor Troicki (SER)
Julien Benneteau (FRA) bt Stanislas Wawrinka (SWI) 6-4,
David Ferrer (ESP) bt Sam Querrey (USA) 7-5, 6-2
(FRA) bt Michael Berrer (GER) 6-4, 6-2