Taylor Fritz opened his Indian Wells title defense with a battling 4-6, 6-4, 6-3 second-round victory Saturday over fellow American Ben Shelton.
Fritz won his fifth ATP Tour title at Delray Beach this year to rise to fifth in the world, the first US man in the top five since Andy Roddick in September of 2009.
But he was on the back foot early against 41st-ranked Shelton, who reached the Australian Open quarterfinals in January in his first trip outside the United States.
"He's a really good player and I expected him to come out just serving balls, playing well – he has nothing to lose so I did expect him to come out with a very high level," Fritz said.
"I just had to regroup, find a way to problem-solve. I'm happy to get through it."
Fritz, a California native, overcame a foot injury last year to beat Spanish superstar Rafael Nadal in the Indian Wells final.
He went on to reach his first Grand Slam quarterfinal at Wimbledon last year.
Seeded fourth in a field led by world No 2 Carlos Alcaraz, Fritz next faces either Australian qualifier Rinky Hijikata or Argentina's Sebastian Baez, the 30th seed.