Third seed Andy Roddick and last year's runner-up Tim Henman survived tough opening matches at the Pacific Life Open Masters Series event.
By contrast, world number one Lindsay Davenport and Wimbledon champion Maria Sharapova cruised to victories in the women's competition.
Roddick was pushed all the way before he edged Spain's Fernando Verdasco 6-3, 3-6, 7-6 in a second round tussle.
Troubled by the left-hander's big forehand, Roddick needed to serve smartly late in the match and played an almost error-free tiebreak to close out the contest.
"That's the biggest forehand I've ever faced," the American told reporters. "He absolutely kills it.
"It would be an amazing shot if he could find a happy medium, rather than just killing it the whole time."
Sixth seed Henman almost wilted in the searing desert temperatures before tightening up his aggressive game in the last two sets to squeeze past Frenchman Arnaud Clement 4-6, 6-4, 6-3 in another second round match.
"It was a struggle certainly to find my rhythm in all aspects of my game," said Henman, who lost to Roger Federer in the 2004 final. "I obviously know I need to play aggressively and take the ball on but I was making so many unforced errors.
"Then you try and rein it in a little bit, you start playing a little carefully and you get involved in his style of game. I just had to keep believing that I could turn it around and get myself to improve and slowly but surely I did."
Playing in only his fourth tournament of the year due to a back problem, Henman wants to get as many matches in as possible.
MAKING PROGRESS
"I've definitely made progress with the way that I feel and obviously the work that I've been doing," he said. "I feel healthy and moving much better on the court."
But compatriot Greg Rusedski was not so fortunate when he fell to 17th seed Fernando Gonzalez of Chile 7-6, 7-6.
"It came down to a point here or there and that's what it's been like for me against the top players this year,' Rusedski said. "Now I just need to start winning those one or two points and I feel like I'm getting closer to doing that."
Ninth seed Andre Agassi brushed aside Australian Wayne Arthurs 6-4 6-1 and there were also victories for fifth seed Guillermo Coria, number 10 David Nalbandian and number 12 Tommy Robredo.
Sharapova wasted little time ousting American Angela Haynes 6-1, 6-2 in the second round to record her ninth straight win, following title successes in Tokyo and Doha.
The Russian teenager now faces 25th-seeded fellow countrywoman Dinara Safina, who defeated American Lindsay Lee-Waters 6-2, 6-4.
World number three Sharapova re-stated her desire to grab the top ranking from Davenport, who put out Sesil Karatantcheva of Bulgaria 6-3, 6-2.
"Why would you want to be number 20 and then when you get to number 20, you don't want to be number one?," said Sharapova.
"It's like 'shoot for the moon'. If you miss, you'll still be among the stars."
Sixth-seeded Russian Nadia Petrova also progressed while 15-year-old Czech Nicole Vaidisova stunned 11th seed Karolina Sprem of Croatia 6-1, 3-6, 6-1.
