Earlier, Caroline Wozniacki powered past Christina McHale of the United States 7-5, 6-0 Monday to reach the third round of the Madrid Open.
The fifth-seeded Wozniacki hit five aces and committed fewer unforced errors on the red clay to beat the 22-year-old player from Teaneck, New Jersey in 1 hour, 30 minutes. Wozniacki will face Agnieszka Radwanska after the Polish player eased past Casey Dellacqua of Australia 6-2, 6-1.
In the men's competition, John Isner of the United States beat Adrian Mannarino of France 7-6 (2), 6-4 to reach the second round.
The 16th-seeded Isner will next play either Jeremy Chardy - another Frenchman - or Thomaz Bellucci of Brazil.
The 25-year-old Johnson from Redondo Beach, California - one of five Americans in the men's main draw in Madrid - will now have a day to recover before facing third-seeded Rafael Nadal of Spain in the second round.
Sam Querrey beat 19-year-old qualifier Thanasi Kokkinakis of Australia 6-4, 6-7 (6), 6-3 - hitting 11 aces in the process, while Feliciano Lopez of Spain overcame Benjamin Becker of Germany 7-5, 3-6, 6-4.
The 11th-seeded Lopez, who lives in Madrid, was given a warning by the chair umpire for using foul language on court during a tense third set.
In the women's tournament, 13th-seeded Lucie Safarova of the Czech Republic bested Tsvetana Pironkova of Bulgaria 6-3, 6-1, Samantha Stosur of Australia beat Kaia Kanepi of Estonia 6-3, 6-2, and 21-year-old Garbine Muguruza of Spain lost to Svetlana Juznetsova of Russia 6-3, 5-7, 7-5.