Pole vaulter Yelena Isinbayeva and hurdler Liu Xiang are enjoying a return to form in Olympic year and head into this week's world indoor championships in Istanbul as event favorites.

Russian Isinbayeva announced she was back to her formidable best with an indoor world record of 5.01 meters in Stockholm two weeks ago following a frustrating three years. That performance from the Olympic champion, whose struggles with form led her to take a break from the sport in 2010, has dampened expectation surrounding American Jenny Suhr, who until Isinbayeva's world mark had the leading clearance of 4.88, and Britain's Holly Bleasdale (4.87).

Such was Isinbayeva's confidence after her world record that when asked if anyone could challenge her this year, she simply said: "I think you can answer that question yourself."

Chinese high hurdler Liu Xiang has struggled with injury since limping out of the 2008 Beijing Olympics with an Achilles problem but the 29-year-old is starting to come good again and set an Asian indoor hurdles record of 7.41 seconds in February.

He will be deprived of a face-off with arch-rival Dayron Robles, however, after the 2010 champion pulled out on Monday with a back injury.