SINGAPORE - Former world number ones Park Inbee and Tseng Ya-ni outshone top-ranked Lydia Ko, 17, as they seized the joint lead on day one of the HSBC Women's Champions on Thursday.

Teenage sensation Ko blamed jetlag and fatigue as she carded four-under-par 68 to lie two shots off Park and Tseng's 66 at a sun-baked Sentosa Golf Course.

New Zealand's Ko deposed Park to become the youngest male or female world number one last month, but the South Korean boosted her bid to wrest back the ranking with a blemish-free round.

The 26-year-old, who bagged five birdies, said: "Today was a really good day. This golf course is a very challenging golf course and I was able to manage myself with a bogey-free round."

She ended the day tied at the top of the leaderboard with Taiwan's Tseng, who reigned as world number one for almost two years between 2011 and 2013 but is now ranked 67th.

Ko started strongly but she looked to be struggling in temperatures which reached 35 degrees C (95 F), and also blamed jetlag after flying in from New Zealand.

"Yeah I'm tired. A little bit jet-lagged because of the long flight," said Ko, who lies tied for fourth after the first round.

"But I've been trying to pace myself, go nine holes and do the least amount of practice."Expectations are sky-high for South Korean-born Ko, who cemented her number one status with back-to-back wins in the Australian Open and then the New Zealand Open last weekend. The HSBC Women's Champions has produced seven different winners in as many years and reigning champion Paula Creamer has work to do if she is to defend her title.Creamer finished the opening day in 51st position with a disappointing two-over-par 74, one stroke behind her compatriot and world number six Michelle Wie.American Angela Stanford, who won the competition in 2012, ended the day on five-under par 67 putting her in third place.Park added that being demoted to second in the world had eased the pressure and allowed her to play a more relaxed game.

 At this week's tournament launch, the affable Korean joked that Ko had "hopefully temporarily" taken over as world number one.