By Dhara Ranasinghe
LONDON (Reuters) - Portugal and Greece, two economies at the heart of the euro zone debt crisis a few years ago, are the bloc's best performing government bond investments of 2017.
A surprise two-notch upgrade of Portugal's ratings into investment grade by Fitch Ratings on Friday caps a stellar performance for a bond market that has seen 10-year borrowing costs more than halve since the start of the year.
Returns on both Portuguese and Greek 10-year government bonds are up around 20 percent each this year, according to Thomson Reuters Datastream.
That is well above returns of 2-4 percent from other euro zone economies including southern European peers Italy and Spain, and benchmark bond issuer Germany.
Portugal, which lost the most money for investors in 2016 on heightened worries about its banks, economy and credit rating, has seen the most marked turnaround.
The once bailed-out southern European state broke ranks with its euro zone peers during the second quarter as an economic turnaround started to make waves with investors.
It is on track for its best full-year expansion in a decade, helped by a sharp reduction of the budget deficit to within European Union guidelines after years of overshooting.
Portugal's latest ratings news means the country now holds an investment grade from two of the three major agencies and could soon return to major bond indices after an absence of over five years.
"Portugal is probably one of the economic success stories of the year," said Nick Gartside, international chief investment officer for fixed income at JP Morgan Asset Management.
"The statistic to zoom in on is the budget deficit. Back in 2015, the budget deficit was 4.4 percent and today it is around 1.4 percent so you can see why Portugal has a well-deserved double-notch upgrade."
At around 1.78 percent <PT10YT=TWEB>, Portugal's 10-year bond yield is down some 200 basis points this year and is trading in line with its Italian equivalent <IT10YT=TWEB>.
While German and Italian bond yields have risen in five and four months, respectively, this year, Portuguese yields have risen in just two -- reflecting strong sentiment towards Portugal at a time when the bond market has seen less support from European Central Bank purchases compared with peers.
Greek government debt, the best-performing euro zone bond investment last year but an illiquid market held by few investors, has continued to deliver stellar returns.
Greek 10-year bond yields are trading comfortably below 5 percent <GR10YT=TWEB> and on Monday hit their lowest levels in around 11 years.
Greece's bond market has also been boosted by a stronger economy, reduced fears about the unity of the single currency bloc and progress in talks with creditors that have boosted expectations that Greece will exit its bailout scheme next year.
"We expect Greece to exit the programme in 2018, but to be subject to ongoing monitoring," Mark Dowding, partner and co-head of investment grade debt at BlueBay Asset Management.
Elsewhere in the euro zone, returns were in the low single-digits, a sign that investors are reluctant to pile into regional bond markets wholesale given that yields remain at relatively low levels.
For a graphic on 2017 euro zone bond market returns, click - http://reut.rs/2kFa5mu
For a graphic on monthly change in yields this year, click - http://reut.rs/2oBkPqZ
For a graphic on Greek 10-year bond yields below 4 percent, click - http://reut.rs/2CUaARV
(Reporting and graphics by Dhara Ranasinghe; editing by John Stonestreet) OLGBBUS Reuters UK Online Report Business News 20171218T125707+0000