World Indoors: Jessica Ennis heads six to watch in Istanbul

Three days of stiff competition begin on Friday as the 14th World Indoor Championships get under way in Istanbul.

The weekend could see Britain's 38-strong squad come away with a medal haul to equal - and maybe beat - the seven it won in Birmingham nine years ago.

Who to watch, when and why? Here are six of the best contests to keep your eyes on.

What: Ennis v Chernova, pentathlon

When: Five-event series begins with 60m hurdles at 0925 on Friday; concludes with 800m at 1735 (all timings GMT).

Why: Istanbul represents reigning world indoor champion Jessica Ennis's first chance to take revenge on Russia's Tatyana Chernova for snatching her World Championship crown in Daegu last summer.

Ennis is in form. At the Birmingham Grand Prix last month she set a world-best of 7.87 seconds in the 60m hurdles and posted an indoor personal best of 6.47m in the long jump. If she can maintain that excellence over all five events then Irina Belova's 20-year-old world indoor record of 4991 could fall.

But Chernova is also in dangerously good shape. She has nailed a big PB of 8.02 seconds in the hurdles this winter, and her combined current PBs in the five events would leave her just 27 points shy of Ennis's own cumulative best.

That neither Ennis nor Chernova are ranked as current indoor number one reflects the strength in depth of the field. Olympic champion Nataliya Dobrynska recently posted a new national indoor record of 4880 points, but it is Russia's previously unheralded Yekaterina Bolshova who may be the surprise package; her 4896 tally last month puts her in prime position to pull off a shock.

What: Mo Farah, 3,000m

When: Heats at 1705 Friday, final at 1310 Sunday.

Mo Farah could not hold his lead at last month's Birmingham two-mile

Why: Farah, Britain's golden boy at the outdoor Worlds last summer will face a severe challenge from the man he beat to 5,000m gold in Daegu, the USA's Bernard Lagat.

At 37 years old, Lagat is now approaching veteran status, but the former 1500m man may still have the speed to deny Farah the indoor crown.

Farah was beaten into second place by Eluid Kipchoge over two miles in Birmingham last month, but he still broke the British and European record with his time of 8:08.07, and Kipchoge will not compete in Turkey.

In his absence, his compatriots Augustine Choge and Edwin Soi - the only men to have broken seven minutes 30 seconds for the 3,000m this season - are the other significant dangers.

What: Men's 60m

When: Heats at 1535 Friday, semi-finals at 1530 Saturday, final at 1800.

Why: Usain Bolt might be missing (the indoors does not suit his long-limbed style) as well as Asafa Powell (failed to qualify), but in Nesta Carter and Lerone Clarke Jamaica will still be favourites to win their first world indoor 60m title.

Britain's Dwain Chambers won gold two years ago in Doha and, despite not being in the 6.48 secs shape that destroyed the field that day, he should improve on his last outing of 6.70 seconds to feature in the medals.

Throw US indoor champion Trell Kimmons into the mix - with 6.45 seconds, the fastest man in the world this year - and you have a showdown which should dominate Saturday evening.

What: Women's pole vault

Bleasdale has shown a stunning level of improvement in recent months

When: Final at 1200 Sunday.

Why: When Britain's Holly Bleasdale smashed the British record with 4.87m January to go third on the all-time indoor list, a first global title at the tender age of 20 seemed a genuine possibility.

It still is. But with world record holder Yelena Isinbayeva dramatically back to near her best with a 5.01m in Stockholm a fortnight ago - her first 5m-plus vault since 2009 - it is the Russian who will start as favourite to grab a fifth world indoor title.

Bleasdale's medal hopes have been boosted by the late withdrawal of USA's Jenn Suhr, the current world number two, but with four other women also over 4.70m this winter the competition will be intense.

A year ago Bleasdale just failed to make the finals of the European Indoors in Paris, going out with a best of 4.45m. That she can be considered a medal contender this time is a mark of how stellar her improvement over the past 12 months has been.

She should also continue to improve, whether or not it happens this weekend. When Isinbayeva was winning her Olympic title in 2008, Bleasdale had yet to even start training for the pole vault. Aged just 20, she has already cleared 22cm higher than the Russian at the same age.

What: Men's 60m hurdles

When: Heats at 0740 Saturday, semi-final at 1245 Sunday, final at 1520.

Why: The late withdrawal through injury of 2010 champion Dayron Robles has denied us the chance to see the Cuban go head to head with his great rival Liu Xiang, but the 2004 Olympic champion seems to be returning to the form that once made him the best in the world.

Liu is now 29 years old, but he turned the clock back to before his disastrous injury at the Beijing Olympics with an Asian indoor record of 7.41 secs in Birmingham in February.

Even with his disqualification for false-starting in Stockholm a fortnight later, he will start as favourite for gold, just ahead of US champion Aries Merritt.

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 Robles, however, after the 2010 champion pulled out on Monday with a back injury.

What: Women's 60m hurdles

Australia's Pearson is in the form to threaten the world record in Istanbul

When: Heats at 1500 Friday, semi-finals 1510 Saturday, final at 1745 Saturday

Why: If Liu is returning to his best, Australia's Sally Pearson is redefining what most thought possible in women's sprint hurdling.

Continuing the blistering form that saw her storm to world gold in Daegu, she clocked 12.49 seconds over the 100m hurdles in wet conditions in Melbourne recently - the fastest ever time by a hurdler at this stage of a season.

Britain's Tiffany Porter has been setting national records of her own and could add to the European Indoor silver she won in Paris a year ago.

But if Pearson can bring her form over from the southern hemisphere then Susanna Kallur's world indoor record of 7.68 secs could be blown away.