On board the Tim Barrett- owned Bradamante - the smallest of the three boats in this year's British Admiral's Cup team - the Olympic silver medal- winners finished second, seventh and fourth to move up two places.
The pair again made a break in the first race and then made sure of consolidating gains in the remaining two as the westerly sea breeze picked up to 14 knots.
"It is always very tight and you have to pick a spot at the turning marks," Merricks said. "But the boat handling is very good and we have quite good pace."
The overall leader after six races is Tomas Friese's I Punkt, helmed by Gavin Brady with John Kostecki as tactician. Behind Merricks and Walker is Tom Roche's Jameson. The gold and silver medal-winning brothers, Torben and Lars Grael, are fifth on Bravo and the reigning world Star Class champion, Enrico Chieffi, partnering his brother, Tommaso, is sixth on Breeze.
The Olympic champion Russell Coutts and silver medallist Jorg Diesch, on Just For Garanta, are one of four German boats vying for their Admiral's Cup team place.
MUMM 36 WORLD CHAMPIONSHIPS (Punta Ala, It) Race 4: 1 V Vascotti (Osama, It), 2 J Merricks/I Walker (Bradamante GB), 3 S Kulmar (Sea Aus). Race 5: 1 T Chieffi (Breeze It); 2 C Larson (Jameson US), 3 G Brady (Tomas I Punkt Ger). Race 6: 1 D Barnes (Georgia Express NZ), 2 Brady, 3 T Grael (Bravo Bra). Overall: 1 Brady 21pts; 2 Merricks/Walker 32, 3 Larson 34, 4 Vascotto 35, 5 Grael 37, 6 Chieffi 41.Reuse content