By Matt Cronin

STANFORD, California (Reuters) - Sluggish Serena Williams struggled through her jet lag to complete a 6-2 6-1 victory over collegiate champion Nicole Gibbs in the second round of the Stanford Classic on Wednesday.

Williams, who picked up the Wimbledon singles and doubles titles at the weekend, was not at her best but served and returned well enough to defeat the 19-year-old American.

"I couldn't quite believe I was playing today," Williams said. "But I'm healthy and I have a heartbeat so I didn't have a reason not to.

"I definitely felt sluggish and it was good to get the win over with, I'm still waking up at 2 A.M."

Gibbs said she had been nervous playing against someone she admired so much.

"“She is such a hero to me and I had her so built up in my mind I thought she was going to do no wrong," Gibbs said. "I collected a lot of experience points out there, if not physical points."

Williams will meet sixth seed Channelle Scheepers in the quarter-finals after the South African earned a 6-3 6-4 victory over Michelle Larcher De Brito of Portugal.

Former world number one Jelena Jankovic continued her sub par 2012 when she was defeated by Coco Vandeweghe 6-4 6-2. It was the Serb's third consecutive first round defeat.

The 20-year-old Vandeweghe scored her third win over a top 20 player. She had lost to the Serbian four years ago, but felt much more comfortable on Wednesday.

"“Her pace isn't as intimidating when I was a little slow and not making that many balls in the court," Vandeweghe said.

"“Having that confidence against someone who moves as well as she does and to keep going for my shots, I thought I did really well today."

Fifth seed Yanina Wickmayer struggled to a 5-7 6-1 6-4 win over Britain's Heather Watson.

"“It was long and really tough," Wickmayer said. “"She didn't make a lot of mistakes. In the second set I told myself to be more aggressive.

"In the third set we were even and it was just a few points between us. Maybe it was experience and a little bit of luck."

(Editing by Peter Rutherford)