Andy Murray won his first match 6-3 6-2 in the San Diego Open after a late replacement saw him face lucky loser Denis Kudla.
The 34-year-old former world number one was set to face Japan's Kei Nishikori, who withdrew with a back injury.
Cameron Norrie also had a straight sets win in the first round over Germany's Dominik Koepfer 6-2 7-5.
British number one Dan Evans is in action on Wednesday and he faces South African lucky loser Kevin Anderson.
Wildcard Murray cruised through his match against the 29-year-old American in just 69 minutes but is likely to face more of a challenge in the second round against the Norwegian second seed, Casper Ruud, ranked world number 10.