India's spin duo of Yuzvendra Chahal and Kuldeep Yadav have been sensational in the three ODIs in South Africa so far. Their combined match-wise figures in these three games are: 5 for 79 in 20 overs (Durban), 8 for 42 in 14.2 overs (Centurion), and 8 for 69 in 18 overs (Cape Town). Together in these matches, they have taken 21 wickets in 52.2 overs, conceding 190 runs - that's an average of 9.04 runs per wicket, and 3.63 runs per over.

These numbers would be impressive in any context, against any opposition; that they have come in South Africa makes them all the more stunning. Admittedly, AB de Villiers and Faf du Plessis are missing (du Plessis played the first game but not the last two), but even so, the fact that Kuldeep and Chahal have delivered these numbers against South Africa in South Africa is hugely creditable. We are only midway through the series, with three matches still to come, but at the moment, these are some of the best figures ever for spinners in an ODI series.

In all series in which spinners from a team have bowled at least 50 overs, there is only one instance of spinners conceding fewer runs per wicket than 10.47, which is the current average for India's spinners in this series. That was by Sri Lanka in Zimbabwe in 2008, when the visitors' spinners, led by Muttiah Muralitharan and Ajantha Mendis, took 27 wickets at an average of 8.22. Muralitharan's 11 wickets cost him 5.81 runs each, at an economy rate of 2.78, while Mendis took 15 at 8.60, conceding 3.58 runs per over.

South Africa isn't usually a place for spinners to be so dominant, which is why the performances of Chahal and Kuldeep are so unusual, and so extraordinary. Spinners from any team have never averaged fewer than 20 in any ODI series in which they have bowled at least 50 overs in this country. The next lowest average is 24.66, by South Africa in the 2016-17 home series against Australia, but they only took 12 wickets then. With the same cut-off, the next lowest average in a bilateral ODI series in England, Australia, New Zealand or South Africa is 17.16, by West Indies against New Zealand in 1994-95.

Best ave for spin in bilateral series in Aus, Eng, NZ, SA (Min 50 overs)
Team Series Overs Wkts Ave
 Ind  Ind in SA, 2017-18  58.2  21  10.48
 WI  WI in NZ, 1994-95  51.5  12  17.16
 NZ  Eng in NZ, 2007-08  57.5  11  18.27
 SA  SA in Eng, 2012  61.0  12  21.17

In fact, the 21 wickets by India's spinners are already the second-highest tally by spinners for a team in a bilateral ODI series in Australia, England, New Zealand or South Africa. The only instance of more wickets was when West Indies' spinners took 24 wickets in South Africa, in a seven-match series in 1998-99. The leading spinners for West Indies then were part-timers Keith Arthurton (12 wickets) and Carl Hooper (nine). With three matches to go in this series, India's spinners will almost surely go past that mark.

For India, the performances of their spinners in this series is also reminiscent of their displays in the 2013 Champions Trophy and the 1985 World Championship of Cricket. Those are the only other instances when India's spinners have averaged less than 20 in a series/tournament in Australia, England, New Zealand or South Africa. In 2013, the leading spinners for India were R Ashwin and Ravindra Jadeja, both of whom have been out of favour since the successes of the wristspinners.

Best ave for Ind spin in Aus, Eng, NZ, SA (Min 50 overs)
Series/tournament Mat Wkts Ave ER Lead spinners
 Ind in SA, 2017-18  3  21  10.47  3.77  Chahal, Kuldeep
 Champions Trophy, 2013  5  20  19.50  4.28  Jadeja, Ashwin
 WCC, 1985  5  18  19.55  3.59  Sivaramakrishnan, Shastri

Wickets in the middle overs have been the biggest area of advantage for India in this series. Twenty of the 21 wickets taken by India's spinners in these three games have come in the middle overs (10.1 to 40), at an average of 10.15, and an economy rate of 3.66. These are incredible numbers in the modern ODI game, when teams look to score at more than five an over even during the middle overs. In bilateral series since the start of 2002, India's spinners have never taken more than 22 wickets in the middle overs of a bilateral ODI series, which means that's another record that will almost certainly fall by the wayside before this series is done. For all teams since 2002, the record in the middle overs of a bilateral series is 26, by Bangladesh's spinners at home against Zimbabwe in 2015-16. In five matches of that series, they took 26 wickets at 15.38.

In terms of averages, the average of 10.15 for India's spinners in the middle overs is currently the best in any bilateral series for spin, with a cut-off of 50 overs. With three matches to go, these numbers will change, and the entry of de Villiers could make things tougher for the spinners in the three remaining matches, but these numbers still show the unprecedented dominance of India's spinners so far in the series.

Best ave for spin in middle overs, bilateral ODI series*
Team Series Overs Wkts Ave ER
 Ind  in SA, 2017-18  55.2  20  10.15  3.66
 Afg  in Zim, 2016-17  58.1  18  12.77  3.95
 Pak  in Ban, 2011-12  68.3  16  13.25  3.09
 Ban  Zim in Ban, 2008-09  51.0  8  13.37  2.09
 Pak  in Zim, 2015-16  61.5  18  13.44  3.91
* Min 50 overs

The numbers for India's spinners look even better because South Africa's slow bowlers have appeared completely toothless against the Indian batsmen. Even as Chahal and Kuldeep have spun rings around South Africa's batsmen, Imran Tahir has figures of 1 for 133 from 24.3 overs, while JP Duminy has 2 for 76 from 12. Overall, South Africa's spinners have returned 3 for 247 from 41.3 overs - that translates into an average of 82.33, and an economy rate of 5.95.

South Africa's spin average of 82.33 is, in fact, their worst, in a home series in which spinners have bowled at least 40 overs. While India's spinners have been setting records for the best numbers, South Africa's spinners - on the same pitches and in the same conditions - are at the other end of the spectrum. The ratio of the spinners' averages for South Africa and India is the fourth largest in any series in which spinners from each team have bowled at least 40 overs. (This excludes the series between Sri Lanka and Zimbabwe, in Zimbabwe, in 1999-2000, when the home-team spinners didn't take any wicket despite bowling 40-plus overs in the series.) These comparative numbers say plenty about the quality of the spinners, as well the quality of the batting line-ups, of the two teams in the ODI series so far.

With inputs from Shiva Jayaraman

S Rajesh is stats editor of ESPNcricinfo. @rajeshstats.