* Nikkei set for worst monthly fall in 2 years * Renesas bounces after recent slide, up 27.5 pct * Olympus gains 4 pct on report of tie-up By Dominic Lau TOKYO, May 30 (Reuters) - Japan's Nikkei average ended a four-session winning run on Wednesday, weighed by exporters as the yen firmed on mounting concerns over Spain's banking system, although Renesas Electronics Corp rebounded after a recent slide. Gains in Softbank Corp, up 2.7 percent after Deutsche Bank upgrade, and scandal-hit Olympus Corp climbed 4 percent, helping to limit the losses on the Nikkei , which closed 0.3 percent lower at 8,633.19. "At the end of the month window-dressing comes into play, but today the buying is mostly just closing out positions," said Masayuki Otani, chief market analyst at Securities Japan. The Nikkei is down 9.3 percent this month, on track for its worst monthly loss in two years, and has fallen 15.8 percent since hitting a one-year peak on March 27 on concerns about the potential impact of a Greek exit from the euro zone and worries over a slowing global growth. "For four of its five weakest May performances, the Nikkei rebounded the following month. A similar pattern holds for November declines - four of the five worst November performances were followed by December rebounds," said Eiji Kinouchi, chief technical analyst at Daiwa Securities. Exporters came under pressure as the yen strengthened to a four-month high against the euro, after Spain's borrowing costs rose on Tuesday towards the 7 percent level that led other countries to seek a bailout. TDK Corp, which has high exposure to Europe, shed 1.6 percent, while Canon Inc lost 0.5 percent. Chipmaker Renesas, whose share price was slashed in half this month, surged 27.5 percent partly on news it would seek 100 billion yen ($1.3 billion) to pay for a proposed restructuring. The stock was the biggest gainer on the main board after sliding 16.4 percent on Tuesday to hit a record low. "It has been oversold ... They're hoping to get some capital injection, significantly lessening the downward pressure," a dealer at a European brokerage said. OLYMPUS UP ON TIE-UP REPORT Olympus rose after the Asahi newspaper said Sony Corp and Panasonic Corp are the leading contenders to take an equity stake in the medical equipment and camera maker. Sony and Panasonic dropped 1.9 and 2.2 percent respectively. The broader Topix index fell 0.5 percent to 723.62. Nearly 1.62 billion shares changed hands on the main board, down from Tuesday's 1.77 billion shares and last week's average of 1.66 billion. The recent sell-off has taken the Topix's 12-month forward price-to-earnings ratio to 10.9, a level not seen since November 2008, Thomson Reuters Datastream showed. Jefferies said it remained long volatility but switched from tactical Nikkei short into short securities and banks as the financial sector was exposed to short-term risk aversion ahead of the Greek elections on June 17. "The Nikkei is nearing support at previous lows of 8,500, there is likely resistance at 8,750, which in current thin volumes, more positive price action may only be short-covering and we may struggle to break higher," Naomi Fink, Jefferies' Japan equity strategist, said in a note. "Until investors bring volume back to the market, upside could be limited." Marubeni Corp sank 3.7 percent after the Japanese trading company bought U.S. grain merchant Gavilon, a deal worth $5.6 billion including about $2 billion in debt, as investors were sceptical of how it would finance its biggest-ever acquisition. The weak commodities market was also a worry for investors, according to analysts, who said it would take time for Marubeni to reap the benefits after becoming the world's largest grain trader following the deal.